ill try and help lol.

f(o)=144 lbs, f(18)=72, therefore f(x)=144-(72/18)x. Next u take the integral of that and taking the integral from 0-18 u get 144x-2x^2 |0-18

giving u an answer of 1944 ftlbs.

im assuming u can take the intrgral by urself so i didnt explain that to much. does that answer ur question?

lol what book are u using, i had the same problem?