36 would go in the question mark box. This is because you square each value from the left side to have it match up with the corresponding right side
7 squared = 7^2 = 7*7 = 49 is the upper left box
4 squared = 4^2 = 4*4 = 16 is the upper right box
5 squared = 5^2 = 5*5 = 25 is the lower left box
6 squared = 6^2 = 6*6 = 36 is the lower right box