The problem is inconsistent. It is missing either a length of one side, or the information that the trapezoid is isosceles.
I suggest to solve the problem considering the isosceles trapezoid, hence the lengths of the two nonparallel sides are equal.
You may consider the lengths of nonparallel sides as x.
The sum of lengths of trapezoid denotes the perimeter of trapezoid.
P = 6 + 14 + 2x => 41 = 20 + 2x => 2x = 41 - 20 => 2x = 21
x = 10.5 cm
The lengths of the other two sides are of 10.5 cm.
The length of the two sides are 10.5cm