The kwon family has a rainwater catchment system they use to water their garden. after 3 days without rain, the depth of water in the tank is 63 inches. after 5 days, the depth is 57 inches. what will the depth of water in the tank be after 17 days?

F(d) = 72 - 3d

if d = 3
f(d) = 72 -3d = 72 - 3(3) = 72 - 9 = 63 inches

if d = 5
f(d) = 72 -3d = 72 - 3(5) = 72 - 15 = 57 inches

so 
if d = 17
f(d) = 72 -3d = 72 - 3(17) = 72 - 51 = 21 inches

answer
the depth of water in the tank will be 21 inches after 17 days