The perimeter is the total measurement of all the sides together. P = 82 Perimeter is side + side + side and so on. So here is what we have. 15.4 + 16.2 + 28.7 + d = 82 Add up what you have now 60.3 + d = 82 Subtract 60.3 from both sides to get D d = 82 - 60.3 d = 21.7
