Catapult Project Need

[Chloers]

I'm doing a catapult project but I'm sort of confused.
I need to find the spring constant in order to get the elastic potential energy.
The force of pulling back the catapult lever to 36 degrees above the horizontal is 4.2 N. Right before the lever is at rest, 90 degrees, the force is 1.4 N.
One of my lab partners got the force average as 6.68 N.
My teacher told me to use the arc length as the x in the equation.
When that was plugged into the equation I got 68.346 N/m as the spring constant. Does that make sense? I asked another group and they got a spring constant of about 2 N/m.

I know I am suppose to use Hooke's Law but am I suppose to use the average force or the difference of the forces?

What am I doing wrong?

 

[mfb]

Without a sketch and other additional information, I think it is impossible to tell.