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 $3\mathrm{.}6$ N $,$ and the string that pulls the combination upward will break if its tension exceeds $7\mathrm{.}8$ 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?

$2$