4x-x^2-21=0

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Solution for 4x-x^2-21=0 equation:


Simplifying
4x + -1x2 + -21 = 0

Reorder the terms:
-21 + 4x + -1x2 = 0

Solving
-21 + 4x + -1x2 = 0

Solving for variable 'x'.

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

Divide each side by '-1'.
21 + -4x + x2 = 0

Move the constant term to the right:

Add '-21' to each side of the equation.
21 + -4x + -21 + x2 = 0 + -21

Reorder the terms:
21 + -21 + -4x + x2 = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -4x + x2 = 0 + -21
-4x + x2 = 0 + -21

Combine like terms: 0 + -21 = -21
-4x + x2 = -21

The x term is -4x.  Take half its coefficient (-2).
Square it (4) and add it to both sides.

Add '4' to each side of the equation.
-4x + 4 + x2 = -21 + 4

Reorder the terms:
4 + -4x + x2 = -21 + 4

Combine like terms: -21 + 4 = -17
4 + -4x + x2 = -17

Factor a perfect square on the left side:
(x + -2)(x + -2) = -17

Can't calculate square root of the right side.

The solution to this equation could not be determined.

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