(x^2-12x−45) is equal to (x+3)(x-15), so the whole expression can be called: ((x+3)(x-15))/(x+3)
The (x+3) can be canceled from the top and bottom, leaving only (x-15).
However, keep in mind that is x is negative 3, then the denominator will be 0, and you can't divide by zero. So it would be more accurate to say the simplified form is (x-15), x!=-3.