Interpreting a force vs. acceleration graph?

Question

I don't understand why im not geting these right! Isn't it just F= ma???

Here is the question: The figure shows an acceleration-versus-force graph for three objects pulled by rubber bands. The mass of object 2 is 0.300 kg. What are the masses of objects 1 and 3?

Here is the figure: http://session.masteringphysics.com/problemAsset/1000350/6/knight_Figure_04_05.jpg

Answer 1

Assume all rubber bands have the same coefficient and are equally streaked therefore produce equal force each.

Yes F= ma and F=-kx n where
n - number of bands
x -distance stretched
k - rubber band constant of( coefficient)

Does that help?

Since object 2 = .300g
3kx= 3a1 (.300)
kx= a1 (.300)
kx/a1=.300

lets do for #1. from the graph
2kx=5.2a1 m1

m1= (2/5.2) kx/a1 and since kx/a1=.300 we have
m1=(2/5.2) (.300)

Answer 2

The slop of a force is 2piR