How do I calculate the Velocity when only given the force and weight?

I do not know how to calculate the velocity of a 50.0g steel ball shot from a slingshot with a 120N/m spring constant when pulled back 35cm and released.

Can someone help me with the formulas to use?

The elastic potential energy is given by $PE=\frac{1}{2}kx^2$
The potential energy will be converted to kinetic energy given by $KE=\frac{1}{2}mv^2$