4.
Using pythagorean theorem:
[tex]\begin{gathered} c^2=a^2+b^2 \\ solve_{\text{ }}for_{\text{ }}a \\ a=\sqrt[]{c^2-b^2} \\ where\colon \\ a=h \\ b=7 \\ c=18 \\ so\colon \\ h=\sqrt[]{18^2-7^2} \\ h=\sqrt[]{275}=5\sqrt[]{11} \\ h\approx16.9yd \end{gathered}[/tex]
