The square of a number is equal to the sum of 21 and 4 times the number. Find the number

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  1. let the number be x

    translate into math

    x^2 = 21 + 4x

    x^2 - 4x - 21 = 0
    (x-7)(x+3) = 0
    x = 7 or x = -3

    the number is either 7 or -3

    if the number is 7
    the square of the number is 49
    21 plus 4(7) = 49 , yeahh!

    if the number is -3
    the square of the number is 9
    21 + 4(-3) = 9, yeahhh!
    Both numbers work.

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