The slope-intercept form:

m - slope
b - y-intercept
The formula of a slope:

substitute the coordinates of the points (8, 0) and (20, 12) to the formula:

y = 1x + b = x + b
Put the coordinates of the point (8, 0) to the equation of a line:
0 = 8 + b subtract 8 from both sides
-8 = b
Answer: y = x - 8