3/12 cancels down to 1/4 because 3 goes into the top and the bottom: 3*1/3*4.
So the sum is now 2/9+1/4. What number does 9 and 4 go into? 36. To get 2/9 so that the bottom is 36 we need to multiply top and bottom by 4: 8/36. To get 1/4 so that the bottom is 36 we multiply top and bottom by 9: 9/36. They both have the same denominator so we can just add the numerators: 9+8=17 so it's 17/36.