Answer
$h=9$ feet
Work Step by Step
We know that the area of a triangle can be calculated as $A=.5bh$, where b is the base and h is the height of the triangle.
We know that the area is 18 square feet and the base is 4 feet, so we can plug these values into the area formula to solve for h.
$18=.5\times4\times h=2h$
Divide both sides by 2.
$h=9$ feet