So first you would want to multiply 84 by 2 because that would let you know how many inches you are using right now. 84 x2 = 168 now that you know you are already using 168 inches you must subtract that from 300 to find out how much you have left 168 - 300 = 132 you now have to divide that by 2 to get how long one side is 132 divided by 2 is 66. She can make it 66 inches wide.