HELP QUICK PLEASE!!!!!!

If a trapezoid is 3 inches tall, it's longest base is 8 inches wide, and the area is 21 inches squared, what is the length of the missing base of the trapezoid?

HELP QUICK PLEASE If a trapezoid is 3 inches tall its longest base is 8 inches wide and the area is 21 inches squared what is the length of the missing base of class=