You have the following information:
- The width of the piece of Plexiglas is 5 cm lower than its length
If you name L as the length of the piece, then, (L - 5) is the width (becasue width is 5 cm lower).
The area of a rectangular figure (as in this case) is given by the product of the width and the length:
area = width x length
you replace in the previous formula for the algebraic expressions, just as follow:
area = L(L - 5)
Hence, the are is given by L(L-5)
