book Maths Puzzle Numerical Ability

cosx+cosy-cos(x+y)=3/2

this is only possible if x=y=60

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book Other Question

This one is also from my worksheet... I appreciate any help. : )
Write a polynomial of least degree which has the indicated zeros.
-1,6i,-6i A price decreased from $36 to $27. What is the percent of decrease?
My answer was 3.24% but I dont think that this is the right answer.
Thanks