This isn't quite enough information. Are we to assume that the base is rectangular? If not what is the angle between sides of the base?
In any case, the volume of such a parallepiped is the area of the base times the height. To get the height, use trigonometry. The third side, with length 20 cm is the hypotenuse of a right triangle with angle 60 degrees. The height is the length of the "opposite side.