So on the left side you have a binomial squared and on the right side you have zero. That's the whole idea. You want to have an equation where you multiply two (or more) things together and get zero. If that is the case, then you know that one of those "things" has to be zero. The only way you get zero is by having one of the factors equal to zero. For instance, 3 times 0 equals 0. 3 times anything else cannot be zero. So, if you have something like (x-4)(x+2)=0 you know that either x-4 is zero or x+2 is zero. If x-4=0 then you know that x=4. If x+2=0, then you know that x=-2.