Hi lemontea.

When factoring, you left out an (Edit: No you didn't; you just canceled it--I'm too tired to read properly). It should be:

Now, remember that , and that , and you should be able to simplify this.

I am assuming you mean and not as it is written. If so, please be more careful about your notation in the future.

Anyway, when rationalizing a binomial denominator, multiply the numerator and denominator of the fraction by the conjugate binomial. For example: