Answer: 
Step-by-step explanation:
You need to remember that formula for calculate the area of a rectangle:

Where "l" is the lenght and "w" is the width.
You know that "x" represents the width of the doghouse, its rectangular base has an area of 21 square feet (
) and its length is 4 feet more than its width (
)
Then, substituting into the formula, you get:

Simplifying, you get the following that can be used to determine the possible dimensions of the base of the doghouse:

