The area is the product of length and width, which is to say that the length and width are factors of the area.
Assuming the dimensions are integers, you want factors of 84 that differ by 5.
... 84 = 1×84 = 2×42 = 3×28 = 4×21 = 6×14 = 7×12
The last of these pairs differ by 5, so ...
- the length is 12 inches
- the width is 7 inches.