The height of the table plus box can vary between 49cm and 57cm (7cm box height plus table height which adjusts from 42cm-50cm). The lowest number in this range that is divisible by 3 is 51. So, for the stack of 3cm books to be able to match the lowest possible height of the box the table must be set at 44cm.
So.
a) the lowest height of the table is 44 (51cm overall height minus 7cm box height)
b) the least number of books is 17 (51cm overall height divided by 3cm book thickness)
Good luck with the rest of your homework!!
;)