Question 291021
Shortest side is X, medium side is X + 7, longest side is X + 11
perimeter is X + (X + 7) + ( X + 11) =3X + 18 = 81
So 3X= 63
X = 21, other sides are 28 and 32 
perimeter is 21 + 28 + 32 = 81 so it checks