How do you subtract add and multiply radicals with numbers in front like

4√ 45-√ 75


And 2√ 5+4√ 20

Try breaking the number inside the radical up into a perfect square.

The second one:

2\sqrt{5}+\sqrt{20}

\sqrt{20}=\sqrt{5\cdot 4}=2\sqrt{5}

See? There is a \sqrt{5} and 4 is a perfect square. So, we can add it to the other.

2\sqrt{5}+2\sqrt{5}=4\sqrt{5}