(a+18)(a-21)=0

Solution for (a+18)(a-21)=0 equation:

Simplifying
(a + 18)(a + -21) = 0

Reorder the terms:
(18 + a)(a + -21) = 0

Reorder the terms:
(18 + a)(-21 + a) = 0

Multiply (18 + a) * (-21 + a)
(18(-21 + a) + a(-21 + a)) = 0
((-21 * 18 + a * 18) + a(-21 + a)) = 0
((-378 + 18a) + a(-21 + a)) = 0
(-378 + 18a + (-21 * a + a * a)) = 0
(-378 + 18a + (-21a + a2)) = 0

Combine like terms: 18a + -21a = -3a
(-378 + -3a + a2) = 0

Solving
-378 + -3a + a2 = 0

Solving for variable 'a'.

Factor a trinomial.
(-18 + -1a)(21 + -1a) = 0

Subproblem 1
Set the factor '(-18 + -1a)' equal to zero and attempt to solve:

Simplifying
-18 + -1a = 0

Solving
-18 + -1a = 0

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

Add '18' to each side of the equation.
-18 + 18 + -1a = 0 + 18

Combine like terms: -18 + 18 = 0
0 + -1a = 0 + 18
-1a = 0 + 18

Combine like terms: 0 + 18 = 18
-1a = 18

Divide each side by '-1'.
a = -18

Simplifying
a = -18
Subproblem 2
Set the factor '(21 + -1a)' equal to zero and attempt to solve:

Simplifying
21 + -1a = 0

Solving
21 + -1a = 0

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

Add '-21' to each side of the equation.
21 + -21 + -1a = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -1a = 0 + -21
-1a = 0 + -21

Combine like terms: 0 + -21 = -21
-1a = -21

Divide each side by '-1'.
a = 21

Simplifying
a = 21
Solution
a = {-18, 21}