You are right. Your formula only works if you were somehow taking the components "with replacement" (like some magic leprechaun replacing each part as you removed it. ).

We need to do this without replacement. This is where good old combinations come in. I assume that order doesn't matter.

The probability of having a good assembly will be the total number of ways of having a good assembly (Q1 below), divided by the total number of ways of making an assembly (Q2 below).

Q1: How many ways are there of making a good assembly? More specifically. how many ways are there to take 6 good parts? You need to take 6 good ones from the 21 good ones.

Q2: How many total ways are there to take 6 parts from all 30?

Can you finish it?