IsNumeric Function in VBA - How to insert a calculation?

I was wondering if anyone had any suggestions for what I'm doing wrong, or if it's even possible.

I am working with 3 columns: Column X = # of Services, Column Y = # of Bills, and Column Z which is a calculation of X divided by Y.

Sometimes, there is a text entry in Column X - when that happens, I want Column Z to simply reflect what Column Y contains.

Here is the code I've written, can't get it to work. I've tried (seemingly) all possible combinations of quotation marks and parentheses to no avail.

ActiveCell.FormulaR1C1 = _
    "=IF(IsNumeric(RC[-2]),=RC[-2]*RC[-1],=RC[-1])"

Thank you!