Find two rational expressions whose difference is equal to -. can somebody explain how to solve it?

We have the following expression:
 - (x + 4) / (4x)
 We must find two rational expressions.
 For this we have, generically:
 (a / b) - (c / d)
 Doing the subtraction of fractions we have:
 (a * d - c * b) / (b * d)
 (-x - 4) / (4x)
 Rewriting:
 - (x + 4) / (4x)
 Thus,
 (-1/4) - (1 / x)
 
 two rational expressions whose difference is equal to - (x + 4) / (4x) are:
 (-1/4) - (1 / x)

It would be 18 cm