Well, you know just by looking at it because it ends in an even number. Like, the greatest common factor of 4 is 2, 1.) Because no other number can "go into it," and 2.) Because it's even.

This is an equation of a parabola that opens upwards.

72*12 = 2*2*2*2*2*3*3*3 = 2^5*3^3