a^2-9a-10=0 Please Solve This for a?

a^2 - 9a - 10 = 0
a^2 + a - 10a - 10 = 0
(a^2 + a) - (10a + 10) = 0
a(a + 1) - 10(a + 1) = 0
(a + 1)(a - 10) = 0

a + 1 = 0
a = -1

a - 10 = 0
a = 10

∴ a = -1 , 10

factor the equation: (a - 10)(a + 1) = 0

set each part = 0: a - 10 = 0
a + 1 = 0

solve for a: a = 10, or a = -1

(a-10)(a+1)

a=10
a=-1