relationalalgebra

Question

Let R = (A, B) and S = (A, C), and let r(R) and s(S) be relations. Write
relational-algebra expressions equivalent to the following domain-relationalcalculus
expressions

{< a > | ∃ c (< a,c > ∈ s ∧ ∃ b1, b2 (< a,b1> ∈ r ∧ < c,b2>∈ r ∧ b1 > b2))}