Solve by Completing the Square x^2-6x=21

$x^{2} - 6 x = 21$

Step 1

To create a trinomial square on the left side of the equation, find a value that is equal to the square of half of $b$ .

${(\frac{b}{2})}^{2} = {(- 3)}^{2}$

Step 2

Add the term to each side of the equation.

$x^{2} - 6 x + {(- 3)}^{2} = 21 + {(- 3)}^{2}$

Step 3

Simplify the equation.

Simplify the left side.

Raise $- 3$ to the power of $2$ .

$x^{2} - 6 x + 9 = 21 + {(- 3)}^{2}$

Simplify the right side.

Simplify $21 + {(- 3)}^{2}$ .

Raise $- 3$ to the power of $2$ .

$x^{2} - 6 x + 9 = 21 + 9$

Add $21$ and $9$ .

$x^{2} - 6 x + 9 = 30$

Step 4

Factor the perfect trinomial square into ${(x - 3)}^{2}$ .

${(x - 3)}^{2} = 30$

Step 5

Solve the equation for $x$ .

Take the specified root of both sides of the equation to eliminate the exponent on the left side.

$x - 3 = \pm \sqrt{30}$

Add $3$ to both sides of the equation.

$x = \pm \sqrt{30} + 3$

Step 6

The result can be shown in multiple forms.

Exact Form:

$x = \pm \sqrt{30} + 3$

Decimal Form:

$x = 8.47722557 \dots, - 2.47722557 \dots$