We have A = {1, 2, 3, 4, 5}. The two sets we're looking at in the problem are:

1st Set: Subsets of A of size 2, so members of this set are things like {1, 2}, {1, 4}, {4, 5}, etc.

2nd Set: Subsets of A of size 3, so members of this set are tings like {1,2,3}, {2, 3, 5}, {3, 4, 5}, etc.

Both of these sets should have size 10 (a good start is to understand why), so bijections are possible.