Try using this:

=((M9472-SUMIF(O2:O9470,"wave",M2:M9470))/(N9472-SUMIF(O2:O9470,"wave",N2:N9470)))*8

It looks like you're doing this because of operator precedence:

         (SUMIF(O2:O9470,"wave",M2:M9470))

(M9472 - --------------------------------- - (SUMIF(O2:O9470,"wave",N2:N9470)))*8 

                N9472