The vertices of a rectangle are always: (x₁,y₁),(x₁,y₂),(x₂,y₂) and (x₂,y₁)
we have:
(x₁,y₁)=(3,3) ;    x₁=3; y₁=3
(x₁,y₂)=(3,6);    x₁=3; y₂=6
(x₂,y₂)=(7,6);     x₂=7,  y₂=6
The fourth vertex will be (x₂,y₁)=(7,3)
Answer:the fourth vertex of this rectanble will be:  (7,3)
         
                 B*(3,6)                    C*(7,6)
                  A*(3,3)                     D*(7,3)