Think What can the numerator and doninator both be divided by? In this case it is 2.
If the number can be divided again think the same question. "What can the numeratoR and doNinator alike be divided by? If it can not be divided aNy further it is in simplest form.
In thsi case it can be dived again, by 3. 12➗3=4 and 21➗3=7. So the answer is 4/7 for it can not be divided again.