Find the area of the polygon with vertices of a(0,1), b(0,5), c(4,5), and d(6,1).

20

If you draw that out on a coordinate grid, you get a trapezoid with one base 6 units long, one base 4 units long and a height of 4. Using the formula for area of a trapezoid 

 h(b1 + b2)/2

where h is the height, 4, b1 is a base, 6, b2 is the other base, 4, we get

4(4+6)/2

which is 4(10)/2 or 40/2

which gets us the answer, 20