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x²-21x=0

2x²+28x=0

-6x+3x²=0

24x=12x²

81x+9x²

x²-21=0

2x²-128=0

-81+9x²

4x²=36

X^2 - 21x = 0

x (x - 21) = 0

x = 0

x - 21 = 0

x = 21

2x^2 + 28x = 0

2x (x + 14) = 0

2x = 0

x = 0

x + 14 = 0

x = - 14

- 6x + 3x^2 = 0

3x^2 - 6x = 0

3x (x - 2) = 0

3x = 0

x = 0

x - 2 = 0

x = 2

- 12x^2 + 24x = 0

x^2 - 2x = 0

x (x - 2) = 0

x = 0

x - 2 = 0

x = 2

x^2 - 21 = 0

x^2 = 21

x = + / - √21

2x^2 - 128 = 0

x^2 - 64 = 0

x^2 = 64

x = + / - 8

9x^2 - 81 = 0

x^2 - 9 = 0

x^2 = 9

x = + / - 3

4x^2 = 36

x^2 = 9

x = + / - 3