How Does Newton's 2nd Law Apply in Different Force Scenarios?

[jactre]

In Figure 5-51a, a constant horizontal force a is applied to block A, which pushes against block B with a 20.0 N force directed horizontally to the right. In Figure 5-51b, the same force is applied to block B; now block A pushes on block B with a 10.0 N force directed horizontally to the left. The blocks have a combined mass of 14.0 kg. What are the magnitudes of (a) their acceleration in Figure 5-51a and (b) force F_a?

Figure is attached

I drew a free body diagrams and came up with:
a) F_a +F_ab = 14a
F_a + 20 = 14a

b) F_a -F_ab = 14a
F_a - 10 = 14a

I don't know what to do from here. Any guidance is appreciated!

 

[iRaid]

30=14a for the first one