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