Answer
$-7\sqrt7$
Work Step by Step
Multiply the radicals to obtain:
\begin{align*}
3\sqrt7-5\sqrt{28}
\end{align*}
Factor the radicand so that one of the factors is a perfect square to obtain:
\begin{align*}
&=3\sqrt7-5\sqrt{4(7)}\\
\end{align*}
Simplifiy the radical by taking out the square root of the perfect square factor inside the radical sign to obtain:
\begin{align*}
&=3\sqrt7-5(2)\sqrt7\\
&=3\sqrt7-10\sqrt7\\
&=(3-10)\sqrt7\\
&=-7\sqrt7
\end{align*}
