Answer :
m^2 + 9m + 20 = 0
(m + 4)(m + 5) = 0
m + 4 = 0
m = -4
m + 5 = 0
m = -5
solution : m = -4 or m = -5
(m + 4)(m + 5) = 0
m + 4 = 0
m = -4
m + 5 = 0
m = -5
solution : m = -4 or m = -5