For part (a) you can modify slightly to get the correct answer.

(4*3*2)/3! * (48*47)/2!

This is also C(4,3) * C(48,2) where C(n,k) is binomial coefficient.

4*3*2 means order matters; 4*3*2/3! means order does not matter.

For part (b) you can do as you suggested, but you will need to modify your expression for 4 aces.