If Statements (?) - If Cell1 =yes then give -Cell2, If Cell1=no then give +Cell2

Hi all - This is probably a pretty basic question (I'm not amazing with excel) but I am hoping someone can help!

Basically, if Colum C= yes then I want the data in Column M to be displayed in reverse signage, in a new column e.g. column N. Any thoughts?

=if(Cx="yes",-Mx,""). Put this in column N. The x represents the row number. I assumed that if C<>yes, then you want a blank in Column N. If this is not the case, then let us know more information
Alan

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