What is the ratio 12/21 equivalent to

4/7, because both the numerator and denominator can be divisible by 3.

Both 21 and 56 are divisible by 7 so, the fraction can be reduced. 7 goes into 21 three times, and 7 goes into 56 eight times, so the fraction is 3/8, which is equivalent to 21/56, and is your answer