7/4 or 1 3/4. First find the LCD, or Least Common Denominator. This number is the lowest number all three denominators can multiply to. Next, take whatever number you multiply to get to the LCD, and multiply it by the numerator and denominator. Then, you just add the numerators and keep the denominators the same. When you get 21/12, you can reduce it to 7/4 by dividing the numerator and denominator by 3.