Ok, I used 0.5 = 0 + 0.5a12 to find a, which gives 1.
So, if F = ma, then the F on the 3kg block is 3(1) which is 3.
On the block suspended in air, the F = mg.
Since we already found F = 3, then 3 = m(9.8), and finally m = .31
This still doesn't make sense, maybe I'm forgetting a T...