Factorize the numerator:
4r + 20 = 4r + 4×5 = 4 (r + 5)
I think you meant to say r ≠ -5, which means r + 5 ≠ 0, so that the denominator is never zero and so the expression is defined (no division by zero). This lets you cancel the factor of r + 5 in the numerator with the one in the denominator:
(4r + 20)/(r + 5) = 4 (r + 5)/(r + 5) = 4
