Ok.

So you have to pick 5 players.

3 Forward from the 12 Forward, that is 12C3=220 sets

2 defence from the 6 Defencemen, 6C2=15

So, in total are 220x15=3300 possible sets of players.

Since only one is on the ice, you have 1/3300 probability for a set of players to be on the ice.