\(\textit{area of a triangle}=\cfrac{1}{2}\cdot b\cdot h\qquad
\begin{cases}
b=\textit{base of triangle,}\\
\qquad \textit{may also be its width}\\
h=\textit{height of triangle}
\end{cases}
\\ \quad \\
b=width=18\qquad area=225
\\ \quad \\
225=\cfrac{1}{2}\cdot 18\cdot h\impliedby \textit{solve for "h"}\)