Answer: 132 square units
Explanation:
Area of a trapezoid:
a = top base; b = bottom base; h = height

Find the bases and the height:
b: 21 (given)
h: 8 (given)
a: find from the bottom
need to find the only not given value by subtracting the values we know. (this might be a little confusing but I think it'll be easier if you look at the original picture)
21 - 9 - 3 = 9
add the new number with the given length of the top base
9 + 3 = 12
Substitute the values into the equation:
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
(I used a calculator to find it out)
Area = 132 square units