To find the "slope", we have to get two points on the graph and find the slope.
The two points on the graph where line A intersects would be at (0,0)
and (4,30).
The slope formula is change in y/ change in x, so....
4 - 0 / 30 - 0 = 4 / 30 = 7.5
7.5 MILES PER HOUR
