2 answers:
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Multiplying fractions is just multiplying the numerators together and the denominators together.
To find out which one is bigger, we need a common denominator. The LCM of 7 and 21 is 21, so convert the first fraction into a denominator of 21. 7 goes into 21 three times, multiply this to the numerator and denominator:
Now just compare the numerators to see which one is bigger.
therefore, 
To find out which one is bigger, we need a common denominator. The LCM of 7 and 21 is 21, so convert the first fraction into a denominator of 21. 7 goes into 21 three times, multiply this to the numerator and denominator:
Now just compare the numerators to see which one is bigger.
Answer:
It's the 3rd one using the formula
Once you follow that step you need to multiply the exponent of 3 and 6 with the exponent outside the parenthesis..... which is why it's the 3rd one.