Forget about the negative sign. First you have to find a least common denominator. This would be 63. Since we had to multiply 9 by 7 to get 63 we have to multiply the numerator (4) by 7. This gives us a new fraction of 36/63. Since we ha e to multiply 21 by 3 to get 63 we have to multiply 19 by 3. This gives us 57. So now we have 36/63 and 57/63. Then we add the negative sign back in making 57/63 -57/63. Now we add them together. Because one has a negative sign and one is positive the number has to be closer to zero. So the answer is -21/63.