Answer
$30\sqrt 6$ feet
Work Step by Step
In part $a)$ we found the equation connecting $l$ and $x$:
$l^2-x^2=225$
Substitute $l=75$ and solve the equation for $x$:
$75^2-x^2=225$
$x^2=75^2-225$
$x^2=5625-225$
$x^2=5400$
$x=\pm\sqrt{5400}$
$x=\pm 30\sqrt 6$
As $x\geq 0$, the only solution is:
$x=30\sqrt 6$ feet
