My interpretation is that you have a base-3 exponential function here, with exponent -3.
-x
3 (-3). If I'm correct here, you meant
-x
-3*3
Is this correct? Once I understand what you started from and what you want to learn here, I can provide more detail.
There's no "formula" for 3-×(-3), Instead, there are rules for evaluating such expressions.
Please write back with more info.