Choose the constant term that completes the perfect square trinomial.

X^2 + 1/3x

To complete the perfect square trinomial, you have to do (b/2) ^2, which in this case is ((1/3) / 2) ^2. 1/3 divided by 2 gives you 1/6 and then (1/6) ^2 is 1/36. So 1/36 is the constant term. This, the complete trinomial is now x^2 + 1/3x + 1/36

Just remember that anytime you need to complete a perfect square trinomial, use the expression (b/2) ^2