I think you mean:
3x / (x + 5) + 7 / (x + 5)
[tex] \dfrac{3x}{x + 5} + \dfrac{7}{x + 5} [/tex]
Since the denominators are the same, the denominators are the common denominator. We add the numerators and keep the common denominator.
[tex] \dfrac{3x}{x + 5} + \dfrac{7}{x + 5} = \dfrac{3x + 7}{x + 5} [/tex]
