calculation using the formula in table

(CALCULATED:(množství * cena *  id_DPH / 100) + (cena * množství)\r\n)
množství (quantity of goods)*cena (price) *id_DPH(value added tax 15% or 21% ) /100 + (cena (price) * (množství)quantity of goods)

1.table pokladna
item:
Název(name goods)
cena(price)
množství(quantity of goods)
id_DPH - relationship 2.table DPH
celkové cena (total price)

2.table
item
DPH(value added tax ->value 15 or 21)

id_DPH -relationship table DPH. Table DPH has an entry: value added tax
id_DPH has value 1 or 2.  I need the value 15 or 21

Please help me