A person can see for a distance of 12 miles. How many square miles can he see? Ok. How many square miles are in one mile?

The last question is incorrect. Square mile is a unit of area (two dimensional), while one mile is a unit of distnace or length (one dimensional). If you need to calculate the observable area (which is a circle, because you observe 12 miles in all directions) then just apply the standard geometric formula

A =π r^{2 }= π 12^{2} = 3.14 x 144 miles^{2} = 452.16 mi^{2}