that's simple the sum of a,b,c is basically saying : a+b+c that it , that's the answer. unless the variable has a corresponding number for example: if a=2,b=3.c=1 a+b+c=6 2+3+1=6
Just simply add the three variables. Since you cannot add different variables, Just like you can't add those with have different units, just leave it as a+b+c. That's it.
