I have two different ways to solve this question. The first method is using the method taught in textbook: Draw the parallelogram and bend F to a direction which is perpendicular to the resultant force, and use sine rule to find F. The second method I use is by assuming the direction of the resultant force of 600N and 400N is parallel to F, hence the resultant force,900N is parallel to 400N and F also. Under this condition, F can only be the minimum. The answer I got from the second method is much smaller than the answer from first method. Can anyone tell me which one is the correct method? Where did I get wrong?? Thanks in advance.