You may know some of the perfect squares, including 4, 9, 16, 25, 36, 49, etc. These are the result of squaring an integer, and their square roots therefore are these integers. You probably know that 4 times 4 is 16. That means 4 squared is also 16, and the square root of 16 is positive or negative 4. The principal square root is just positive 4.
So you could write 4, -4, and 4 in the boxes.