First thing you want to do is find the common denominator for \frac{1}{3} and \frac{3}{4}. You do this by multiplying to denominators together, so 3*4=12.
Next, multiple the numerators with the opposite denominator. 1*4=4 and 3*3=9.
Now you are left with \frac{4][12] and \frac{9}{12}.