Slope-intercept form: write an equation from a graph

Read slope and intercept from a graph and write y = mx + b.

Lesson 5

Concept

Find b where the line crosses the y-axis, then compute m from any two points. Substitute into y = mx + b.

A line crosses the y-axis at 3 and passes through (2,7). Write its equation.

A line with slope 4 and y-intercept -1 is:

y = 4x - 1

y = -4x + 1

y = 4 - x

y = x - 1

If b = 5, which point must lie on the line?

(5,0)

(0,5)

(1,5)

(5,1)

These steps are out of order — can you fix them?

1.Locate y-intercept

2.Compute slope from two points

3.Substitute m and b into y=mx+b

4.Check with another point

What should you check after writing y = mx + b from a graph?