If the base is 7.2 and the top is 6.4, you know that the extra bit on the bottom is (7.2 - 6.4 =) 0.8. This means on each side the base of the imaginary triangle is 0.4
Using Pythagoras' theorem, we know a² + b² = c²
0.4² + 3.6² = c²
c = √0.16 +12.96
c = √13.12
c = 3.62 -> this is the diagonal length of one of the sides of the shape.
So the distance all around is 7.2 + 6.4 + 3.62 + 3.62 = 20.84 inches