Answer
$5\ ft$
Work Step by Step
Step 1. Based on the figure given in the exercise, use Pythagorean Theorem, we have:
$h^2+12^2=(2h+3)^2$ which gives $3h^2+12h-135=0$
Step 2. Factor $(h-5)(3h+27)=0$ and solve the equation (discard the negative answer) to get $h=5\ ft$