To solve this problem we can use pythagorean theorem, this way:
In this case, the height is one of the legs of the triangle, use the pythagorean theorem to find it:
[tex]\begin{gathered} h=\sqrt[]{15^2-5^2} \\ h=\sqrt[]{225-25} \\ h=\sqrt[]{200} \\ h=14.1 \end{gathered}[/tex]The answer is 14.1 ft.