Perimeter is just adding all the lengths of all the sides. So You already have the two sides which are 28 each. If you add those two is 56. So now you would subtract 56 from 86 since 86 is your total.
86 (perimeter) -56 (total of two sides)
This gives you 30
Since you have two equal sides left, you will divide 30 by two and that gives you 15
