I can factor it?

Geometry Level 3

$\large \cos^2( x) + \cos^2( 2x) + \cos^2 (3x) - 1 = a \prod_{k=1}^b \cos( kx)$

If the equation is true for all values of $$x$$ with constants $$a,b$$ ,what is the value of $$2a+b$$?

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