Let the two forces be F1 and F2 and let F2 is smaller than F1 and assume R is the resultant force. Given F1+F2=18 .....(i) From the right angle triangle, F22+R2=F12 or F12−F22=R2 or (F1+F2)(F1−F2)=R2 or (18)(F1−F2)=(12)2=144 or (F1−F2)=8.....(ii) By solving equation (i) and (ii) we get, F1=13N and F2=5N