First evaluate the sq root of 45. To do so, you have to find perfect square factors (2 x 2, 3 x 3, 4 x 4, and etc.)
The factors of 45 are 3 x 3 x 5.
Notice that there is a 3 x 3. That means you can get the 3 out of the square root and multiply with the number outside of the square root.
11(3)square root 5 - 4 square root 5
33 square root 5 - 4 square root 5
Subtracting and Adding square roots is the same thing as subtracting and Adding variables. If they have the same variable you can +/-. So if they have the same square root, then can +/-.