Hmm, I'll take the question that Nacha asked. I'll take the why axis as the line of reference for the measurements of angle. And, I'll take the angle from the why axis to the 600 N force to be 45 degrees for ease of work.
So, we need to find the resultant of the 600 N (45 degrees from why axis) and 400 N (-60 degrees from why axis) forces.
Using parallelogram law of forces, you'll get a figure similar to that:
Well, you will need to find the 'length' (magnitude) of the resultant (double arrowhead). You can find this by using the cosine rule and taking the two sides of the triangle with sides 400 N, 600 N and resultant.
The angle between the 400 N and 600 N is found to be 75 degrees ([45 + 30] degrees, or [180 - (60+45)] degrees)
Now, the cosine rule:
A = ?
B = 600
C = 400
Solving, you'll get A = 629.1 N
Now, to find the angle, I'll use the sine rule.
Here, angle a is 75
A is 629.1
B is 600
Angle b = ?
From the sine rule, you get angle b = 67.1 degrees.
But you know that the angle between the why axis and the 400 N force is 60 degrees and the angle in the triangle is more than that. So, the angle from the why axis is [67.1-60] = 7.1 degrees.
Hence, resultant force = 629.1 N, 7.1 degrees (anticlockwise) from why axis.