Take two from the left to the middle, the left is 2 * 2 = 4 less than the middle, the right is the middle 1, and the middle is 5 more than the left. Now take the middle and left numbers, leaving 5.

You can also use the equation, assuming that the beginning of each pile is x-2 on the left and x+2+ 1 in the middle.

(x+2+ 1)-(x-2)=5