Quadrilateral abco is a rectangle give the coordinates of b without using any new variables

We know that
point A and point B
have the same x coordinate
so
x coordinate of B is a

and point C and point B
have the same y coordinate
so
y coordinate of B is c

therefore 
the coordinate of B is (a,c)

the answer is
B (a,c)

Agreed i also had the same answer