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what specifically about mechanics? do u have an engineering based question?

Nah, general question.

Are you an engineer ?

|dw:1444139335225:dw|

nah, just ok at maths

if we use the Law of Cosines on that triangle
|dw:1444139454924:dw|

That's why I was getting the wrong angles -,-

How ? It wasn't clear 131 ? on the last question

Using the Law of Cosines idea?
|dw:1444139889064:dw|

|dw:1444139666557:dw|

Why isn't 180 - Alpha = alpha ? Symmetry

|dw:1444140291240:dw|

yes. with theta= 40, you get 120-alpha
now it will work

*140 - alpha

300/ sin alpha = 256.9 / sin40
sinalpha^-1 = 48.64 -,-

What am I doing wrong ?

I think it would help if you drew a picture that more closely matched the situation

|dw:1444140790074:dw| That's our start then |dw:1444140826725:dw| then using the sine rule.

|dw:1444141166027:dw|

the other way around the ambiguity is choose the acute angle rather than alpha

140 - alpha ?

Alpha doesn't seem like an obtuse in the drawing :/

|dw:1444141658047:dw|

Fine, thanks phi :)

\[ \frac{\sin (140-\alpha)}{60}= \frac{\sin(40)}{256.9}\]

Phi, I noticed that 256.9 is out of the choices -,-

a)112.7 b ) 145.9 c) 154.3 d ) 257.9

Yes, I noticed that before. I assume it is a typo.
Here are the 3 vectors

Ok, as you're sure.

You can ask your teacher, but the numbers work out to 256.9

Yeah, 257.9 works too with 131.4

Yes, I'm sure the magnitude is 256.9
But complain to your teacher.

I will.