Let A1 be
and A2 be
$\text{In the question it is given that A1 ⊆ A2}$
$πA1(πA2(σC1(σC2(R1))))→πA1(σC2(σC1(R1))$
$\text{It is the implication given to you, let LHS be A and RHS be B}$
$\text{This clearly means A→B which means ¬A V B}$
$\text{Which clearly says this is gonna be true in each case except for if A is true and B is false}$
So As A1 ⊆A2 this is always a true statement. Hence true