The slope of a line that passes through points (x1, y1) and (x2, y2) is computed as follows:

The line of the picture passes through (0, 20000) and (40, 0) then its slope is:

The y-intercept of the line is (0, 20000)
Slope-intercept form of a line:
y = mx + b
where m is the slope and b is the y-coordinate of the y-intercept. Replacing with m = -500 and b = 20000, we get:
y = -500x + 20000