12/63 simplified is 4/21 because...
What numbers go into 12 and 63? 3! So, how many times does 3 go into 12? 4! So 4 will be our numerator, or top number. How many times does 3 go into 63? 21! Therefore, our denominator, or bottom number, will be 21. You will know that this is in the simplest form because no other number goes into 4 and 21 other than 1.
Your answer is 4/21.
Hope I helped! :)
~Cher_Bear~