Help with a mechanical energy exercise

[Felafel]

Homework Statement

A spring cannon is used to shot horizontally a marble mall, whose mass is 75g, from a platform located 1.2 m from the ground. If the spring compression is 25 mm, the ball hits the ground 4,2m from the base of the platform. Not taking friction into account, determine:
1-the mechanical energy of the ball during the motion
2-the "k" constant factor of the spring
3-how far the ball would hit the ground if the compression were 37mm.

The Attempt at a Solution

With no friction, i can use the principle of conservation of energy

I want to compute the mechanical energy for the top of the platform.
$E_i=E_f$ $\Rightarrow$ $U_i+K_i=U_f+K_f$
But $K_i = 0$ because the ball is not moving yet, and $U_f=0$ So,
$U_i-K_f=E$ $\Rightarrow$ $E=\frac{1}{2} k x^2 - \frac{1}{2} m v^2$

However, I keep having two variables: velocity and k, and I don't know how to find the value of any of the two, even using other formulas. Can anyone give me a hint?

 

[ap123]

When the ball leaves the platform, you know the distance it moves in the x- and y-directions.
So you can work out its speed at this point.
That will help you with the rest of the problem.