7x-x^2=21

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Solution for 7x-x^2=21 equation:


Simplifying
7x + -1x2 = 21

Solving
7x + -1x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 7x + -1x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 7x + -1x2 = 0

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

Divide each side by '-1'.
21 + -7x + x2 = 0

Move the constant term to the right:

Add '-21' to each side of the equation.
21 + -7x + -21 + x2 = 0 + -21

Reorder the terms:
21 + -21 + -7x + x2 = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -7x + x2 = 0 + -21
-7x + x2 = 0 + -21

Combine like terms: 0 + -21 = -21
-7x + x2 = -21

The x term is -7x.  Take half its coefficient (-3.5).
Square it (12.25) and add it to both sides.

Add '12.25' to each side of the equation.
-7x + 12.25 + x2 = -21 + 12.25

Reorder the terms:
12.25 + -7x + x2 = -21 + 12.25

Combine like terms: -21 + 12.25 = -8.75
12.25 + -7x + x2 = -8.75

Factor a perfect square on the left side:
(x + -3.5)(x + -3.5) = -8.75

Can't calculate square root of the right side.

The solution to this equation could not be determined.

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