Spring Potential Energy

1. Homework Statement

A horizontal spring with stiffness 0.6 N/m has a relaxed length of 18 cm (0.18 m). A mass of 23 grams (0.023 kg) is attached and you stretch the spring to a total length of 25 cm (0.25 m). The mass is then released from rest. What is the speed of the mass at the moment when the spring returns to its relaxed length of 18 cm (0.18 m)?

2. Homework Equations

Kf + Uf = Ki + Ui
((1/2)mv^2)$$_{f}$$ + ((1/2)KsS^2)$$_{f}$$=((1/2)mv^2)$$_{i}$$ + ((1/2)KsS^2)$$_{i}$$

3. The Attempt at a Solution

I know that the final Uf should be zero since the final length of the spring would be zero. I am confused on what I should plug in for the velocities, one of them should be left as a variable and the other perhaps set to zero?
Please let me know what I'm doing wrong, and set me on the right track!