Need help with physics question!

Question

I attempted this problem but I don't where to get the k constant from to complete the equation.

1/2(1.2+0.01)V^2=1/2kx^2

A rifle bullet with mass 10.00g strikes and

embeds itself in a block with mass 1.2

kg that rests on a friction less, horizontal surface

and is attached to a coil spring. The impact

compresses the spring 15.0cm. Calibration

of the spring shows that a force of 0.850N is required to compress the spring 0.250cm.

What is the velocity of the bullet?

Answer 1

The spring constant k = 0.85 / .0025 = 340 N / m