(a-9)(a+1)=0

Solution for (a-9)(a+1)=0 equation:

Simplifying
(a + -9)(a + 1) = 0

Reorder the terms:
(-9 + a)(a + 1) = 0

Reorder the terms:
(-9 + a)(1 + a) = 0

Multiply (-9 + a) * (1 + a)
(-9(1 + a) + a(1 + a)) = 0
((1 * -9 + a * -9) + a(1 + a)) = 0
((-9 + -9a) + a(1 + a)) = 0
(-9 + -9a + (1 * a + a * a)) = 0
(-9 + -9a + (1a + a2)) = 0

Combine like terms: -9a + 1a = -8a
(-9 + -8a + a2) = 0

Solving
-9 + -8a + a2 = 0

Solving for variable 'a'.

Factor a trinomial.
(-1 + -1a)(9 + -1a) = 0

Subproblem 1
Set the factor '(-1 + -1a)' equal to zero and attempt to solve:

Simplifying
-1 + -1a = 0

Solving
-1 + -1a = 0

Move all terms containing a to the left, all other terms to the right.

Add '1' to each side of the equation.
-1 + 1 + -1a = 0 + 1

Combine like terms: -1 + 1 = 0
0 + -1a = 0 + 1
-1a = 0 + 1

Combine like terms: 0 + 1 = 1
-1a = 1

Divide each side by '-1'.
a = -1

Simplifying
a = -1
Subproblem 2
Set the factor '(9 + -1a)' equal to zero and attempt to solve:

Simplifying
9 + -1a = 0

Solving
9 + -1a = 0

Move all terms containing a to the left, all other terms to the right.

Add '-9' to each side of the equation.
9 + -9 + -1a = 0 + -9

Combine like terms: 9 + -9 = 0
0 + -1a = 0 + -9
-1a = 0 + -9

Combine like terms: 0 + -9 = -9
-1a = -9

Divide each side by '-1'.
a = 9

Simplifying
a = 9
Solution
a = {-1, 9}