first, notice that the first three terms, x^2-12x+36, is a perfect square, and can be factored into (x-6)(x-6),, now rewriting the expression, it becomes:
(x-6)^2 - 49y^2
(x-6)^2 - (7y)^2
this looks like difference of two squares,, let a = x-6 and b=7y, thus:
a^2 - b^2
so there,, :)
Gotta remember those damn steps. I immediately assumed it was four terms so I factor by grouping. I forgot that you first look for common factor, then squares etc.