According to the PEMDAS rule, the correct answer would be 21.
First, you multiply 6 and 7 and get the product of 42.
Then, multiplying again but with different numbers, 7 and 3, you will get 21.
Lastly, subtract the two numbers, 42 and 21 and you will get the difference of 21 as your final answer.
The correct answer is 21.
