Question

If four times a number is added to the square of the number, the result is 21.find the number​

Answer 1

n = 3, n = -7

Explanation:

let n = the number we are looking for

4n + n² = 21

rearrange in the equation to

n² + 4n - 21 = 0

by factoring

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

n₁ = -7, n₂ = 3

substitute both to find out which one gives the correct answer.

4n + n² = 21

4(-7) + (-7)² = 21

-28 + 49 = 21

4n + n² = 21

4(3) + (3)² = 21

12 + 9 = 21

∴ n = 3, n = -7