Write in 0 and 0 at the top: this indicates that if there are zero pennies, the height of the tower is 0. This is the point (0,0). Another point is (4, 28): a tower of 28 pennies is 4 cm tall.
Find the slope of the line connecting (0,0) and (4,28).
4cm - 0 cm
m = ----------------- = (1/7) (cm/penny)
28 - 0
The equation we need, relating # of pennies to height of tower, is
t = (1/7) (cm/penny)x, where x is the # of pennies.
Thus, if x = 1, then t=1/7 cm
And, if x =15, then t = 15/7 cm
And so on.