You start driving north for 12 miles turn right

you have traveled in the form of a right triangle where the vertical leg is 12 miles and the horizontal leg to the right is 16 miles.

your direct route back is the hypotenuse of this right triangle.

that makes the distance back equal to square root of (16^2 + 12^2) = 20 miles.

that's because pythagorus says that the length of the hypotenuse of a right triangle is equal to the square root of ((the length of one leg) squared plus (the length of the other leg) squared).