Plugin:fetch field vaule from two fields,1st field on one form(entity), second field on other form ,compute it and reflect vale on the field present on second form.

I am new to CRM,

I have a requirement where i need to write a plugin to retrive value :

suppose i have two entity forms,

taking into consideration value from field 1 (Present 2nd entity form) and field 2(Present on 1st entity form) and suppose i multiply 1 and 2 fields and compute the result to show on field 3 present on 2nd entity form