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

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.

c.

step-by-step explanation:

because first you have to add the 4x, 9x, and the 7x together and you get 20x and then you add the last numbers up and you get 5and then you put the subtraction sign in there and that's how you get the answer.