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
The highest point over the entire domain of a function or relation is absolute maximum whereas lowest point under the entire function is absolute minimum....