Here we are given the ratio 12 / 21.
In order to find its equivalent ratio with lower terms , we have to find common factor of 12 and 21 by which we can divide both the numbers.
So factors of 12 are 1, 2 ,3, 4.
Factors of 21 are 1, 3, 7.
So our common factor is 3 here.
dividing both numerator and denominator by 3 we have:

12 divided by 3 equals 4 and 21 divided by 3 equals 7.
So our new ratio becomes 4/7