According to my calculations, To find the perimeter of a rectangle you use Pythagorean theorem. A squared + B squared = C squared. Your C is the diagonal and your side is A. So to get your unknown on one side you will subtract A squared from both sides leaving you with B squared = C squared - A squared. Now lets say for example your numbers are we'll say C= 5 and A= 3. The number 5 squared is 25 and 3 squared is 9, 25-9= 16. Now you will take the square root of both sides leaving you with B= 4. Now that you know your two sides to find the perimeter you need to multiply the height times two plus the width times two, for this example you would have 6 + 8= 14.