The perimeter of a rectangle may be found by either of 2 formulas. The formulas are P = 2L + 2W and P = 2(L + W). If you know 2 of the 3 numbers you can find the 3rd number. For instance, if you know the perimeter and the length you can find the width. The length, width and perimeter must all have the same units.
A rectangle has a perimeter of 84 and its length is 20. Using the first formula, the problem becomes 84 = 2 * 20 + 2W which becomes 84 = 40 + 2W which becomes 44 = 2W, so W = 22. Using the 2nd formula the problem becomes 84 = 2(20 + W) which becomes 42 = 1(20 + W) or 42 = 20 + W which becomes W = 22. Easy.
In each problem of this problem set you will be given the perimeter and then the length, such as 40 & 6. You will need to use a formula to find the width. I highly recommend using the 2nd formula. Good luck and enjoy the challenge!