-x^2+8x=21

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Solution for -x^2+8x=21 equation:


Simplifying
-1x2 + 8x = 21

Reorder the terms:
8x + -1x2 = 21

Solving
8x + -1x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 8x + -1x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 8x + -1x2 = 0

Begin completing the square.  Divide all terms by
-1 the coefficient of the squared term: 

Divide each side by '-1'.
21 + -8x + x2 = 0

Move the constant term to the right:

Add '-21' to each side of the equation.
21 + -8x + -21 + x2 = 0 + -21

Reorder the terms:
21 + -21 + -8x + x2 = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -8x + x2 = 0 + -21
-8x + x2 = 0 + -21

Combine like terms: 0 + -21 = -21
-8x + x2 = -21

The x term is -8x.  Take half its coefficient (-4).
Square it (16) and add it to both sides.

Add '16' to each side of the equation.
-8x + 16 + x2 = -21 + 16

Reorder the terms:
16 + -8x + x2 = -21 + 16

Combine like terms: -21 + 16 = -5
16 + -8x + x2 = -5

Factor a perfect square on the left side:
(x + -4)(x + -4) = -5

Can't calculate square root of the right side.

The solution to this equation could not be determined.

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