To find y = mx + b you need to determine the slope. To find slope use the formula
m = y2 - y1 / x2 - x1
Use two points on the graph such as (4, 100) and (12, 300)
Now plug it into the formula
m = 300 - 100 / 12 - 4
m = 200 / 8
m = 25
Now we know the slope, but we need to find the y-intercept.
y = mx + b
y = 25x + b
Substitute one of the points (4, 100) as the y value and x value to find b, the y intercept
100 = 25(4) + b
100 = 100+ b
100 - 100 = b
0 = b
Now plug in your formula
Y = 25x + 0
y = 25x