Question

$21x = 196 - {x}^{2}$

Answer 1

21x=196-x²
x²+21x-196=0
x²+(28-7)x-196=0
x²+28x-7x-196=0
x(x+28)-7(x+28)=0
(x-7)(x+28)=0
x=7 or x=-28

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