### Home > AC > Chapter 10 > Lesson 10.4.2 > Problem 10-139

10-139.

Use the quadratic formula.

Add 4 to both sides.

Solve for *x*.

*x* = −5

Complete the square.

Subtract 21 from both sides.

*x*^{2} − 10*x* = −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*.

*x* − 5 = 0

*x* = 5