The length of the rectangle is 4cm more than its width if the area of the rectangle is 45cm, what is its width?

Assign a variable to the rectangle's sides.

If the length of the rectangle is l, then the width would be l-4, right?

The area is 45 cm^2, and the area is obtained through the product of the length and the width, this gives:

l\ \times\ (l-4) = 45

Can you solve this to get the length of the rectangle? Then, you have enough information to guess the width :)