Find points on the function f(x) = 2sinx + sin²x for when the tangent line is horizontal.

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so i found the derivative of that function, which is

f'(x) = 2cosx + 2sinxcosx

what do i do now? set f'(x) to 0? i tried that, but the answer key says that the answer is

(π/2 + 2nπ, 3), (3π/2 + 2nπ, -1)

what does that notation mean? i am confused.. thx