a^2 - b^2 = (a-b)(a+b)
This formula is sufficient to help you solve all the problems. The first one is pretty straight forward, it is already in the difference of square form. THe second problem has a common factor of (a) so you can factor that out first, and then it will be in a difference of square form. As for the third one, you need to realize that 4x^2 is a difference of square (it is (2x)^2) and (y-2z)^2 is also obviously a square.