Alright, the next one is rationalize and cancel.
Let's take this equation:
As you can see, the answer when you do direct substitution is 0/0, and that's not what we want.
Now. This is where most of the mistakes are made. What you want to do, is get rid of the square root, so to do that, you must multiply the numerator's equation by a fractional 1. Numerator and denominator must be the OPPOSITE equation as the numerator. It's hard to explain, so here's a picture. (as shown):
This leaves you with a new numerator. Don't multiply out the denominator.
As you can see, you can now cancel out the (x-4) from both numerator and denominator. After another direct substitution, you get: 1/4