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
