Since you have a common denominator, just add the numerators and set the sum over the common denominator.
[tex] \dfrac{15}{7x} + \dfrac{12}{7x} = \dfrac{15 + 12}{7x} = \dfrac{27}{7x} [/tex]
There is no common factor of 27 and 7x, so the fraction cannot be reduced.
Answer: C. [tex] \dfrac{27}{7x} [/tex]
