Find the sum of the following.x 6by 3 + (-7)by 6 + (-9)by 12

Dear Student, 
 
   We have, $\frac{6}{3}+\frac{(-7)}{6}+\frac{(-9)}{12}$
  Solving, we get, 

    $$\begin{gathered} Taking LCM, we get, \\ =\frac{4\times 6-2\times 7-9}{12} \\ =\frac{24-14-9}{12} \\ =\frac{1}{12} \end{gathered}$$

Hope this information clears your doubts.

Regards