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