how to add fraction with different denominator

Here is a number of sites that cover this topic.  Why don't you take a look.  :)

https://www.google.com.au/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=adding%20fractions%20with%20different

First thing you need to do is to make the denominator the same.

you do it by making the denominator as their LCM (Lowest Common Multiple)

since the LCM of 7 and 6 is 42,

$${\frac{\left({\mathtt{1}}{\mathtt{\,\times\,}}{\mathtt{6}}\right)}{\left({\mathtt{7}}{\mathtt{\,\times\,}}{\mathtt{6}}\right)}}{\mathtt{\,\small\textbf+\,}}{\frac{\left({\mathtt{1}}{\mathtt{\,\times\,}}{\mathtt{7}}\right)}{\left({\mathtt{6}}{\mathtt{\,\times\,}}{\mathtt{7}}\right)}} = {\frac{{\mathtt{13}}}{{\mathtt{42}}}}$$  

 hope it helped :)