Divide it into chunks of area you can find. One way to divide it is
.. a rectangle 2 mi x 5 mi at upper left.
.. a rectangle 8 mi x 6 mi down the middle
.. a rectangle 3 mi x 4 mi at lower right
.. a triangle 5 mi x 6 mi at lower left
Then the sum of areas is
.. (2 mi)*(5 mi) +(8 mi)*(6 mi) +(3 mi)*(4 mi) + (1/2)*(5 mi)*(6 mi)
.. = 10 mi^2 +48 mi^2 +12 mi^2 +15 mi^2
.. = 85 mi^2