How do you find the length of sides of trapezium?

Which side lengths? To calculate the parallel sides, you need the height of the trapezium and one of the sides, and you substitute them into the formula:

h(a + b)/2, where h = height, a and b are the parallel side lengths.

If you want to find the sides that are not parallel, you need the parallel sides, as well as the height of the trapezium. Then, by using Pythagoras theorem, with the side length the hypotenuse, you can find their lengths.