y=x^2-4x+4-36 is another way of expressing this (by completing the square). This is the same as y=(x-2)^2-36. When y=0 x-2=±6 which identifies the x intercepts as x=2+6=8 and x=2-6=-4. The axis of symmetry lies midway between these two points, when x=2, which is the equation of the axis of symmetry. As you can see, it's equidistant (6 units) from the x intercepts. The vertex is on this line, and y=-36 when x=2, so the vertex is at (2,-36).