a
Free body diagram of block is shown below.

Now, from the energy conservation,

\(w=\Delta K\)

\(w_{F}+w_{s p}=\frac{1}{2} m v^{2}-0\)

\(\Rightarrow F_{x}-\frac{1}{2} k x^{2}=\frac{1}{2} m v^{2}\)

\(\therefore v=\sqrt{\frac{2 F_{x}-k x^{2}}{m}}\)