How to solve c^2 - 4c = 21

C² - 4c = 21

Subract 21 from both sides to get all terms on one side of the equation.

c² - 4c - 21 = 0.

Now we factor. We must find two factors that when multiplied equal - 21 but when added together equal - 4. The two factors are - 7 and 3.

c² - 7c + 3c - 21 = 0

Now factor.

c (c - 7) + 3 (c - 7) = 0

These are your two expression to solve.

(c + 3) (c - 7) = 0

Set each equal to 0 to find your answers for C.

c + 3 = 0

c = - 3

c - 7 = 0

c = 7

C = - 3, 7