Answer
a) $f(81)= 50.4$
b) $x= 144$
Work Step by Step
Given $$f(x)=7.2 \sqrt{x}. \tag{1}$$ a) Set $x= 49$ in equation $(1)$ and determine $f(49)$. \begin{equation}
\begin{aligned}
f(49)&=7.2 \sqrt{49} \\
&=7.2\cdot 7\\
&=50.4.
\end{aligned}
\end{equation} b) Set $f(x)=86.4$ in equation $(1)$ and solve for $x$: \begin{equation}
\begin{aligned}
f(x) & =86.4 \\
7.2 \sqrt{x}& =86.4 \\
\sqrt{x} & = \frac{86.4}{7.2} \\
\left(\sqrt{x}\right)^2& = \left(12\right)^2\\
x&=144.
\end{aligned}
\end{equation}
