STANDARD DISCLAIMER:  YOU are responsible for anything you submit to an instructor.

"READ IT BEFORE YOU FEED IT" TO THEM.

If the perimeter of rectangle is 13cm and its width is 2 3/4 (or 2.75) cm, to find its length. just subtract (2.75 x 2), or 5.5, from 13 to get 7.5 cm, the remaining total perimeter.  Divide by two and you get 3.75 (or 3 3/4) cm per equal side.

Do it backwards to double check.  "x" is the perimeter:

Simplifying

(2.75 * 2) + (3.75 * 2) = x

Multiply 2.75 * 2

(5.5) + (3.75 * 2) = x

5.5 + (3.75 * 2) = x

Multiply 3.75 * 2

5.5 + (7.5) = x

5.5 + 7.5 = x

Combine like terms: 5.5 + 7.5 = 13

13 = x

I think we have a match!