Frankly I'd express these in lowest terms before I subtracted. This isn't necessary, but makes the numbers a little nicer. So....
The LCM of 4 and 3 is 12. So we want the denominator in each fraction to become 12. For the that means multiplying the denominator by 3. But what we do in the denominator we must do in the numerator as well:
Similarly for we need to mulitply the denominator by 4:
I'm going to reorder this so that I'm adding whole numbers to whole numbers and adding fractions to fractions. (Or you could just express these as improper fractions and add the fractions.)
Again I'm going to put in lowest terms:
Now add the fractions. The LCM of 4 and 2 is 4. Thus we need do nothing to the first fraction. To make the denominator in the second fraction 4 we multiply it by 2. So: