5\sqrt{8} and 6\sqrt{2} look not like like radicals.

For the square roots, factor the radicands all the way down to perfect root and prime numbers. And then transform them to like radicals.

5\sqrt{8}-6\sqrt{2} = 5\sqrt{4\times 2 }-6\sqrt{2} = 10\sqrt{2}-6\sqrt{2}=4\sqrt{2}