Functional equation hint. How to start?

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If $$f(x)-2f(1-x)=x$$ Solve $$f(\sin x + \cos x)=\frac{\sqrt{2}-4}{6}$$

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Write $f(1-x)-2f(x)=1-x$ and solve the system.

I got $x=15^{\circ}+360^{\circ}k$ or $x=75^{\circ}+360^{\circ}k$, where $k\in\mathbb Z$.