Problem with forecasting (diferent lengths)

Hi, i´m currently forecasting sales knowing the sales and promotion already. But i´m having some problems. I have to turn the variable sales into stationary to make the rigth preditions. To do that, i applied a difference (d=1).
Now i dont know how to foracast sales with the price and promotion beacuse the length from sales are different from price and promotion. How can i forecast this? thank you for the help.

To solve your immediate problem, remove the first row from your explanatory variable, price and promotion.

But, be warned that running a regression on a differenced variable with explanatory variable in levels is likely to have insignificant results. Plot the variable to see why.

I would recommend that you difference the price and promotion as well, in which case, you wouldn't have to remove the first row. Or maybe you should as all 3 variables will have an NA in the first row.

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