Answer
$V_a-V_b = 3.3~V$
Work Step by Step
We found that the current in battery 2 is $0.33~A$ and the direction of this current is up.
We can find $V_a-V_b$:
$V_a = V_b + \mathscr{E}_2-i_2~R_2$
$V_a-V_b = \mathscr{E}_2-i_2~R_2$
$V_a-V_b = 4.0~V-(0.33~A)(2.0~\Omega)$
$V_a-V_b = 3.3~V$
