Use the quadratic formula.

$x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$

Add 4 to both sides.

$x=\frac{-10\pm\sqrt{10^2-4(1)(25)}}{2(1)}$

Solve for x.

$x=\frac{-10\pm\sqrt{100-100}}{2}$

$x=\frac{-10}{2}$

x = −5

Complete the square.

Subtract 21 from both sides.

x² − 10x = −25

Draw a generic rectangle.

You need 25 to complete the square.

Complete the rectangle.

Factor.

(x − 5)² = −25 + 25

(x − 5)² = 0

Solve for x.

$\sqrt{(x-5)^2}=\sqrt{0}$

x − 5 = 0

x = 5