Clara writes the equation (x – 13)(x + 8) = 196 to solve for the missing side length of a rectangle represented by the factor x + 8. What is the missing side length represented by x + 8 units of the rectangle?

Let the side represented by x + 8 be y. Then x − 13 = y − 21, so the equation becomes y(y − 21) = 196, or y² − 21y − 196 = 0. Factoring gives (y − 28)(y + 7) = 0; a side length must be positive, so the missing side length is 28 units, choice D.

AI-written and checked against the verified answer.