You're basically looking for the form (x-a)(x-b)
This is because in your equation, the coefficient x (-15) is negative but the constant, 36 is positive.
So when you expand (x-a)(x-b) you get:
x$^2$ - ax - bx + ab which is the same as:
x$^2$ - (a - b)x + ab. If we compare the coefficients in that to the equation
x$^2$ - 15x + 36
you will see that we need two numbers such that
-a-b=-15 and
a*b=36
The two numbers are 12 and 3
So x$^2$ - 15x + 36=(x-12)(x-3)
