We want to get rid of the roots in the denominator (rationalize the denominator).

The denominator is square root of 11 plus 8 and we want to use difference of two squares rule so we multiply by square root of 11 minus 8.

Multiply in the second step by square root of 11 plus 8 (instead of negative 8) and you will still be still stuch with a square root in the nominator:

Keep on mind that you can obtain an explanation for any step by clicking on it. Please note that you cannot click on the very first expression, which is the original problem.