Explanation:
a) The slope formula is given by
m = (y2 - y1)/(x2 - x1)
Let P1(7, 5) and P2(21, 15)
m = (15 - 5)/(21 -7)
= 10/14
= 5/7
b) Let's see if the slope is only a third of the one found in part (a) or m = 5/21. Let P1(0, 0) and P2(21, 5).
m = (5 -0)/(21 - 0)
= 5/21
Indeed, the slope is only a third of part (a), as we predicted.