I'm having difficulty solving a subnet allocation homework problem.
Assume that a university has an address block 128.205.224.0/19. It has to allocate addresses for 2 departments' networks, each of size 1800, and for 4 offices, of sizes 550, 600, 650, and 750 nodes respectively. Assuming that the university network allocates addresses sequentially from the beginning of the allocated allocated address space, what are the prefix allocations for these subnetworks?

I first thought in this way:
There should be 6 subnets in the network. So I need 3 bits for the subnets. So 3 + 19 = 22 bits should be the network bits. Then there are only 10 bits left. 2^10 = 1024 < 1800, so this cannot work.

Could you guys give me a hint or some thoughts for solving this problem?