Answer
$1944$ ft-lb
Work Step by Step
We know that Hooke's Law states that $F=k x$
To calculate the work done, the limits for $x$ will be from $0$ to $18$ such that:
$W=\int_0^{18} (144-4x) dx=[ 144x-4(\dfrac{x^2}{2})]_0^{18}$
or, $W=144(18-0)-2(18^2-0)=2592-648 =1944$ ft-lb
