Finding Attributes using Relational Algebra

Question

I have a couple of questions that I'm unsure of. Here goes...

The schema:
Instructor(InsID, I_Name,DeptName,Salary)
Student(SID,StName,DeptName, Tot_Credit)
Course(Course_ID,Course_Title,DeptName, credits)

a. Find the course titles for all 3-credit courses offered by the Science dept.
b. Find the student IDs for all students who studied with the Instructor Johnson

I tried to answer them, but I'll need someone to verify/help me find the correct answer.
a. (Pi)Course_Title(Course|X|((sigma)(credits=3 and DeptName="Science")(Course)))

b. (Pi)SID((sigma)(I_Name="Johnson")(Instructor))

I'm not sure if those are correct, but any help would be appreciated.

Answer 1

a is correct but can be shorter. You have 3 ops, only two are needed. b is incorrect (but I'm not going to spoonfeed anything more).