Substitute the given values for $x$.

$g\left(8\right)=\sqrt{8-7}=\sqrt{1}=1$

$g\left(32\right)=\sqrt{32-7}=\sqrt{25}=5$

After seeing the first two done for you, attempt the third one on your own.
If you do not get a perfect square, approximate your answer to the nearest hundredth by using a calculator..