For trapezoids, the area is:
(B1+B2) x H
-------------------
2
B1 = one of the two sides that are parallel to each other
B2 = the other of the two parallel sides that are equal to each other
H = the height of the shape
So for problem 1, it would look like:
(7+9) x 5
-------------
2
And that would end up being 40 cm^2
For problem 2, it's
(5.7+4.7) x 3.2
--------------------
2
And for problem 3, it's
(5.8+7.4) x 3.9
---------------------
2
Hope that helps!