## How to Write Linear Equations

Read,5 minutes

An equation presented in slope-intercept form is written as \(y \ =\ mx \ + \ b\)

Where \(m\) is the **slope** of the line and \(b\) is the \(y\)-intercept. If you know the slope and the \(y\)-intercept, you can use this equation to write a line equation.

### Example

Find the line's equation.

**Solution:**

We should find the \(b\)-value and the \(y\)-intercept, to do that, so we select two points on the line:

Now, determine the **slope **between the \(2\) points:

\(m \ = \ \frac{y_2 \ - \ y_1}{x_2 \ - \ x_1} \ = \ \frac{0 \ - \ 3}{-1 \ - \ 0} \ = \ \frac{-3}{-1} \ = \ 3\)

The \(b\)-value can be found by just looking at the graph:

\(⇒ \ b \ = \ 3\)

Now, we can use the equation \(y \ =\ mx \ + \ b\) and write the line's equation:

\(y \ =\ mx \ + \ b \ ⇒ \ y \ =\ 3x \ + \ 3\)

In a lot of cases, the value of \(b\) isn’t read so **easily**. In that case, or in cases where you are **not** sure if the line really crosses the \(y\)-intercept at this particular point, \(b\) can be calculated by solving the equation for \(b\), then substituting \(x\) and \(y\) with one of the two points.

The previous example can be used to explain this:

- The two points are \((-1 \ , \ 0)\), and \((0 \ , \ 3)\). We determined the slope from these two points: \(m \ = \ 3\)
- Now we have the equation: \(y \ = \ 3x \ + \ b\)

From that the equation can be solved for \(b\): \(b \ = \ y \ - \ 3x\) - Also, if we input the values from the second point \((0 \ , \ 3)\), we have: \(b \ = \ 3 \ - \ 3(0) \ = \ 3\)
- Finally, we can write the line equation: \(y \ = \ 3x \ + 3\)

### Summary:

Writing linear equations:

**Step1:**Determine the slope, \(m\). This can be done by utilizing the slope**formula**to get the slope between two known points on the line.**Step2:**Determine the \(y\)-intercept. In order to do this, simply**substitute**the slope and the \((x \ , \ y)\) coordinates of a point on the line in the slope-intercept formula and then calculate \(b\).**Step3:**Once you have \(m\) and \(b\), you can enter them into the slope-intercept equation and find the line equation.

### Exercises for Writing Linear Equations

**1) **Find the line equation of the line passing through \((10,9)(11,0)\)\( \ \Rightarrow \ \)

**2) **Find the line equation of the line passing through \((8,7)(9,-5)\)\( \ \Rightarrow \ \)

**3) **Find the line equation of the line passing through \((7,10)(8,-6)\)\( \ \Rightarrow \ \)

**4) **Find the line equation of the line passing through \((9,5)(10,3)\)\( \ \Rightarrow \ \)

**5) **Find the line equation of the line passing through \((10,2)(11,3)\)\( \ \Rightarrow \ \)

**6) **Find the line equation of the line passing through \((1,4)(2,2)\)\( \ \Rightarrow \ \)

**7) **Find the line equation of the line passing through \((7,3)(8,5)\)\( \ \Rightarrow \ \)

**8) **Find the line equation of the line passing through \((2,9)(3,5)\)\( \ \Rightarrow \ \)

**9) **Find the line equation of the line passing through \((3,6)(4,-4)\)\( \ \Rightarrow \ \)

**10) **Find the line equation of the line passing through \((5,1)(6,-3)\)\( \ \Rightarrow \ \)