Answer
$72900$ 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 $180$ such that:
$W=\int_0^{180} (4.5x) dx=4.5(\dfrac{x^2}{2})]_0^{180}$
or, $W=2.25[(180)^2-0]=2.25 \times 32400=72900$ ft-lb