Answer:
It will cost $ 3348 to carpet the lobby.
Explanation:
Refer the attached figure
AB divides the figure into two congruent trapezium
Height of trapezium = 15.5 feet
Parallel sides = 30 feet and 42 feet
Area of one trapezium =

Area of one trapezium =

=

So, Area of both trapeziums =

So, Total Area of the floor is 1116 sq.feet.
Cost of carpet of 1 sq.ft. = $3
Cost of carpet of 1116 sq.ft. =

Hence it will cost $ 3348 to carpet the lobby.