Question

If the sum of the square of the number and 4 time the number is 21,
what is the number? ​

Answer 1

I think there is a missing “and” there: “If the sum of the square of the number and 4 times the number is 21, what is the number?”

If so, then we have: x² + 4x = 21, or x² + 4x - 21 = 0.

This factors as (x+7)(x-3) = 0, so x = -7 or x = 3.