Conditional formating on sum of 2 cells

jsiegs  if the cell A36 is going to be the difference in A34 and A35 then yes as Bennadeau  mentioned make A36 

=A34 - A35

and there are preset conditional formatting options you can use.

But, if A36 is something else entirely and you only want the conditional formatting on that cell and not the formula, you can do that too.

In conditional formatting you have the option for a custom formula and enter 

=(A34-A35)<0  and define the formatting to be red fill

you can then either make the default fill green by formatting the cell that way or create a second conditional formatting for =(A34-A35)>0 and define it with green fill

If you need more help or have problems I'd be happy to give a sample.