To find the area of this trapezoid, we need to find the length of altitude DE. Let’s call
this altitude h. See the next figure.
Since this is an isosceles trapezoid, the base angles are equal and so ADE and BCF are congruent. Thus, AE and BF are of equal length. Let’s call this length x. Since the length of AB is 14 and EF DC 8, we have
Now we can look at AED; since it is a right triangle, we can apply the Pythagorean theorem.