Write a relational algebra expression that returns total amount of money spent by Sean.

The problem I am having with this question is that if you want to find the total amount of money that Sean spent, you have to multiply the Quantity column from Order relation and Price column from Store relation. How can do it?

Customer Order CID orderNumber CIDsID Model 123 C01 Hd12 124 C02 Rf07 125 C02 Rf07 126 C01 Rf07 cName Quantity C01 CO2 C03 Store Aline Sean John FB409GMBK E55-F1 FB409GMBK E55-F1r Product SID Hd12 FB409GMBK Rf07 Rf07 Tr08 Model E55-F1 FB409GMBK Graco LiteRider Model Price 150 460 150 TV fitBit stroller Categor Electronics Electronics Bab E55-F1 FB409GMBK Graco Lite Rider 110