Let's analyse this. What do we know?

the two bases: 50 and 70.

the height: 7.5

So what trapezoids does it define? We can draw two parallel lines 7.5 inches apart:

The bases lie on this lines. We can fix one of the bases (let's fix the larger one) and move another one to imagine all possible trapezoids that fit the requirements:

We already should understand that the sum of the lengths of sides is not the same for all such trapezoids. But if we do NOT have such understanding and still have doubts, we can take two specific well-known trapezoids:

Draw some heights for easier comprehension:

Then we can easily calculate the sides using Pythagorean theorem:

As wee see, the perimeters in the two cases do differ.