The formula for the area of a trapezoid is:
[tex]A=\frac{1}{2}(b_1+b_2)\cdot h[/tex]where b_1 is the length of one base, b_2 is the length of another base and h is the height.
In this case, we have the following information:
[tex]\begin{gathered} b_1=3mi \\ b_2=11mi \\ h=3mi \end{gathered}[/tex]then, using these values on the formula, we get:
[tex]\begin{gathered} A=\frac{1}{2}(3+11)\cdot(3)=\frac{1}{2}(14)(3)=\frac{1}{2}(42)=\frac{42}{2}=21 \\ \Rightarrow A=21mi^2 \end{gathered}[/tex]therefore, the area of the trapezoid is 21 square miles