I would be willing to talk about this if in exchange you explain in a very detailed way what exactly you don't understand about this problem. I find it hard to believe that you don't know how to solve 3a(a - 1) = 0 after you've been shown how to solve x(x - 7) = 0 and have been told that a product is zero if and only if one of the factors is zero. It may be that you don't know what a factor is or you don't know how to solve linear equations. I'd like to know what your reasoning is before further explanations.
Okay... I can see that the left hand side must equal to the right hand side which is 0. I don't know how you get from the quadratic to the point of finding out what x or a could be. I don't know what the factors are and so don't understand where to put the 0 in either a or b. In fact, i don't know where a or b are in x(x - 7) or 3a(a - 1).
Ah, now we are talking. The left-hand side, 3a(a - 1), is a product of three numbers: 3, a and a - 1. Here a is some number that we don't know yet and have to find. Each of those three numbers, i.e., 3, a and a - 1, are called factors. Now, multiplication has this property that if the product equals zero, then one or more of the factors also equal zero.