What is the number of square units in the area of trapezoid abcd with vertices a(0,0), b(0,-2), c(4,0), and d(4,6)?

The number of square units is what called "area" . The area of a trapezoid can be computed as :

where b is the length of the short basis, B is the length of the long basis and h is the Height. 
If you place the vertices of the trapezoid, you'll notice that b=2, B=6 and H=4. 
Is it clear to you ?