The width od a rectangle is 6 inches shorter than 3 times its length. the width of the rectangle is at most 45 millimeters. what is the length of the rectangle

So the width can be written as W=3L-6. (L is length and W is width). The maximum the width could be is 45mm, so try to use 45mm in place of W. 45=3L-6. Add 6 to both sides and you get 51=3L. Divide both sides by 3 and you get 17=L, where L is the length.