First, it is important to know the prices of each of the instruments in order to determine the number of djembe drums you had sold. Since it is not given here. I will give a general equation where you just substitute the prices. It would be as follows:
326 = 12x + 6(y) + (xx)z
where x is the price for a set of maracas y is the price for a set of claves xx is number of djembe drums sold z is the prices for each djembe drum
Because when you go behind the decimal, it starts out with tenths, then hundredths, and then thousandths. And 6 in the 1st spot behind the decimal so it's tenths.