Write a relational algebra expression that returns customer name that never spent more than 100 on their order.

The problem I have with this question is that you have to compare the values of customers to 100. Also, you have to multiply quantity column of order relation to Price column of Store relation. Here's my implementation:

_cName- _cName(_Price>100(Customer Order Store))

Is it correct because I didn't multiply quantity to price?

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