Answer:
The room has a length of 27 feet and width 15 feet.
Explanation:
Given that:
Area of den = A = 405 square feet
Let,
L represent the length of room and W represent the width of room.
L = W+12
Area = Length * Width

Factorizing the equation

Either,
W-15 = 0
W=15
Or,
W+27=0
W=-27
As the width cannot be negative, therefore,
Width of den = 15 feet
Length of den = 15+12 = 27 feet
Hence,
The room has a length of 27 feet and width 15 feet.