This can be solved using simultaneous equations. Please refer to the diagram for the labelling.
From the diagram we can see (by Pythagoras' Theorem):
Subtracting the first from the second,
Then, subtracting  from ,
So we have
Wow...I'm so confused -_-
Ok, could you please tell me which part confused you? Do you understand the diagram, have you learnt Pythagoras' Theorem?
What happens to the other 2 sides that aren't labelled?
You can make them be in the same triangle :P, since what you have here is a rectangle, the opposite sides are equal.
Thanks, that clears up everything, I understand it now
And I think what I did previously is kind of similar to the process you posted