Shop has to produced 30 furnitures.
It must be at least 9 dinner tables for 120$ and 13 coffe tables for 150$.
We noticed that income will be greater when shop will produce as much is possible coffe tables.
So if sum of furnitures is 30, shop should produce 9 (minimum) of dinner tables and the rest of cofee tables.
There are 9 dinner tables and 30-9=21 coffe tables.
Income is equal:
120*9+21*150=4230$