Projectile Motion spring launch

[BoydC1996]

Homework Statement

I am trying to find the force needed from a spring to launch a mass of 800g to a length of 0.875m. This launch would be at the optimum angle of 45º. I am aware of all projectile motion equations but am having difficulty linking them to force and thus solving the problem itself.

Homework Equations

v^2=u^2+2as, v=u+at, s=ut+1/2at^2

The Attempt at a Solution

 

[CWatters]

You need to break the problem into two parts.

Part 1: Use projectile motion equations to calculate the required launch velocity.
Part 2: Use spring equations to work out how to achieve that velocity over whatever length of spring compression is available.

You don't appear to have enough information in the problem statement.