A marketing campaign aims to increase sales of a product by 8% per month. If this goal is met, within 7 months the sales will reach 2570 units per month. About how many units per month were being sold before the start of the campaign?

Try this:

8% of 2570 is 2570*0.08=205.6 units

and since the number of units were increasing every month you can multiply that by seven giving you 1439.2, then you can subtract that by your final amount of sales giving you the initial amount of 1130.8 units.