algorithm – how to solve operations in txt file c++?

I got a file that named “Input.inp” and it will be opened with filestream and it will be read. Then operations will be done. But i dont know how to do it. E.g.

[Input.inp] File includes;

A = 5
B = 7
C = A*B

I know all the other operations about filestream or anything else, just wondering how to make it done in txt file. How to make it solve?