D is partly constant and partly varies with V. When V = 40, D = 150, and when V = 54, D = 192. Find D when V = 73.

D = mV + C, (compare this to your familiar y = mx + b)

when V=40, D=150

150 = 40m + C * *

when V=54, D=192

192 = 54m + C * **

subtract * * from * **

42 = 14m

m = 3

sub into * *, 150 = 120 + C

C = 30

so you have D = 3V + 30

sub in your given value of V