What is the Correct Method to Solve this Trigonometry Problem?

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Not homework just my own revision.

If sin a = –cos b = 3/5 and a and b are both in the second quadrant, what is cos (a – b)?

Now keep getting the answer 0, but the answer is apparently 24/25, now they use the trig subtraction formula, I just did cos ((arcsin(3/5) - arccos(-3/5)) I got 0 as the answer.

Was my method wrong, where did I go wrong with it?
 
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Both arcsin and arccos are multivalued. You need to get the possible solutions and see which are in the second quadrant.
 
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mathman said:
Both arcsin and arccos are multivalued. You need to get the possible solutions and see which are in the second quadrant.

Ah that's what the explanation was talking about.
 

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