Answer
$5+5\sqrt{3}$
Work Step by Step
Distribute $\sqrt5$:
$\sqrt{5}(\sqrt{5}+\sqrt{15})$
$=\sqrt{5}\sqrt{5}+\sqrt{5}\sqrt{15}$
Recall the property (pg. 367):
$\sqrt[n]{a}\cdot \sqrt[n]{b}=\sqrt[n]{ab}$ (if $\sqrt[n]{a}$ and $\sqrt[n]{b}$ are real numbers)
Applying this property, we get:
$=\sqrt{5\cdot5}+\sqrt{5\cdot15}$
$=\sqrt{25}+\sqrt{75}$
$=\sqrt{25}+\sqrt{25\cdot3}$
Using the property again, we get:
$=\sqrt{25}+\sqrt{25}\sqrt{3}$
$=5+5\sqrt{3}$ (because $5^2=25$)