As shown in the figure, two blocks are connected by a very light string, and the upper block is ...

As shown in the figure, two blocks are connected by a very light string, and the upper block is pulled upward by a different string.  The masses of the upper and lower blocks are 300 g and 240 g, respectively.  The string between the blocks will break if its tension exceeds 3.6 N, and the string that pulls the combination upward will break if its tension exceeds 7.8 N.
(a) What is the largest upward acceleration that the blocks can be given without either string breaking?
(b) If the upward acceleration is slightly higher than this, which string breaks, the upper one or the lower one?