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.