The average of three numbers is 24. if two of the numbers are 21 and 23, what is the third number?

Points j and k plotted on the coordinate grid are two vertices of rectangle jklm rectangle jklm has an area of 7 square units point j is located at 2,1 and point k is located at -5,1 each vertex of the rectangle is located at a point that has integer coordinates. what is the perimeter of rectangle jklm? (need answer asap)