need help to design calculator function (addition and multiplication)

The result of your multiply operation needs a 3 decimal digits representation. I guess, you don't want to create a table (or case structure) with 511 entries for all 21 segments?

So you would want to implement a signed binary to BCD to decoder and then apply 7-segment decoding to each digit. I further guess, that you won't be assigned to a complex exercise without some previous lectures dealing with the involved mathematical operations or at least respective literature links.

My suggestion is to try the decoding by pencil and paper first and than think about a Verilog HDL representation.