I am stuck on relation agebra

I can do for example

Find the Last Name and Tel. Number of all staff who are working as manager.

Rel1= SELECT FROM Staff WHERE position=manager.
Rel2=BRANCH NATJOIN Rel1 with BN common.
Rel3= PROJECT Rel2 OVER LastName, TelNumber

But am stuck when there is more detailed query such as the one below

1.Find the name (first name and surname) of the customer that have been fixed by mechanic "M111" and results have not been successful.

THE DATABASE

Customer (CustomerCode, CustomerSurname, CustomerFirstname, Sex, Age,Ocupation, Address)

Mechanic (MechanicCode, MechanicSurName, MechanicFirstName, Address, PhoneNo, Function)

Service (Service Code, CustomerCode, Mechanic Code, Date, Time, Result, ServiceType)

Is_Seen_By (CustomerCode, MechanicCode, Date, Time).



Question:
Now am stuck as to the relation algebra to find the name (first name and surname) of the customer that have been fixed by mechanic "M111" and results have not been successful