the diagonals of a parellelogram are 20 cm and 30cm long and intersect at an angle of 25 degrees. Find the sides of the parellelogram?

i used the law of cosines to find one of the sides but i am getting the wrong answer.i let sides a=10 and b=15 and angle C =25 and used the law of cosines got wrong anwser. Where am i going wrong