Answer:
x = 3 or -7
Step-by-step explanation:
Recall that for a quadratic equation in the form:
Ax² + Bx = C
we need to add (B/2)² to both sides of the equation to
In our case, we are given:
x² + 4x = 21
compared to the general equation above, B = 4, hence we need to add (4/2)² to both sides
x² + 4x = 21 (complete the square)
x² + 4x + (4/2)² = 21 + (4/2)²
x² + 4x + 2² = 21 + 2²
(x+ 2)² = 21 + 4
(x+ 2)² = 25 (take square root of both sides)
(x+2) = ±√25
(x+2) = ±5
hence
(x+2) = +5----> x = 5 - 2 = 3 (answer)
or
(x+2) = -5 ----> x = -5 - 2 = -7(answer)
