Answer:

Explanation:
We have been given that the mirror is 3 feet wide and the thickness of the frame is x, so width of the frame would be
.
The width of mirror with frame would be
.
Similarly, the length of the frame would be
and the length of mirror with frame would be
.
We know that area of rectangle is width times length, so area of frame with mirror would be
.
We are told that The mirror and frame must have an area of 15 square feet, so we can equate our equation with 15 as:

Upon simplifying our equation, we will get:




Therefore, our required equation would be
.