The perfect square is a number that is the product of two identical numbers. So 9 is a perfect square because it's the product of 3 * 3. For variables, perfect squares can be expressed as

or

, where a is some number and x is the variable.
If we expand these terms,

, and

. Since the given term is

, we can see that it's part of the perfect square

. We just need to find

.
So,

. Notice that

, so

. So, the missing constant term is

. The perfect square is

.