Question

what should be added to thrice the rational number - 7/4 to get 3/7

Answer 1

Let’s get a common denominator first. To do this multiply all the denominators together. You will hear about “least common denominators” but those are a waste of time. In this case, however, the easy common denominator and the LCD are the same. 4*7=28

So we have -49/28 and 12/28

Thrice (quaint) -49/28 is -147/28

Now get the difference (subtract)

12/28 - (-149/28)

Subtracting a negative is the same as adding a positive

12/28 + 149/28 is 159/28

If you need it as a mixed number (mostly useless IMO since to use it further you have to convert it back to an “improper” fraction - really terrible name) it is

5 19/28