If we draw another line inside the trapezoid half way between the top and bottom lines. This line will cut the sides on the left and right exactly in half.
And that is exactly what is happening in this picture. The side to the right is perfectly divided in half and the side to the left is also divided in half by that dashed line. That means that the second upper half of trapezoid is also 3, which makes the height to be 6.
Now that you know the height, you can find out the area. (17+7)/2*6 = 72 cm^2
