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I have no idea how to do this
I'm sorry you missed the classes where they covered this. To make up, you can try online lessons like these
for learning how to solve inverse trig equations. Once you've got some of the basics, then please try this problem. Keep in mind that the inverse sine is some angle (call it A) and the inverse cosine is another angle (call it B) so the (say) cosine of the whole thing is cos(A + B) = cos(pi/4) = 1/sqrt(2). And you can do identities
to simplify the sum.