The perimeter of a rectangle is 34 units. its width is 6.5 units.

It´s length is 10.5 because:

Perimeter = (2 x width) + (2 x length), so the equation is:

Length = [Perimeter - (2 x width)] / 2

20y5 times 2 is 1010 times -2 is -20, plus y