Hey guys and girls I have a maths problem I'm stuck on I keep seeming to get the wrong answer when I move on to work out the angle of the triangle. Attached is the question and my working sheet in an image format. If anyone could tell me where I've gone wrong and help that would be great!

Thanks in advance.

Like I said it is when using the cosine rule to work out the angle that I struggle and seem to get the wrong answer.