A hint to get started
Select indistinguishable beads by colour cases
A box contains 7 beads which are indistinguishable except for their colour, of which 3 are red,2 are blue,1 is white and 1 is green. Given that the order in which they are selected does not matter, how many ways can 3 beads be selected?
CJC · Lecture · Example 11
Decide whether order matters and repetition is allowed; then count stages or disjoint cases.