Start with the given expression
In order to add these two fractions, these fractions need to have a common denominator.
In order to do that, we simply find that the LCM of 8 and 12 is 24 (note: if you need help with finding the LCM, check out this solver)
Now we need to get each denominator to 24
Multiply by 
Multiply and to get 
Multiply by 
Multiply and to get 
Since both fractions have a common denominator of 24, we can now combine the fractions
Combine the fractions
Add the numerators
So

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