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?