After getting 9n2=12q2, we can simplify to 3n2=4q2. This only tells us that n is divisible by 2 (which we already know since p=3n is divisible by 2) and q is divisible by 3 (Since 3 is not divisible by 4, q2 is divisible by 3. And since 3 is prime, q must be divisible by 3)