Help plz

I am not very good at trig but I will try to solve it.

$(cos\space x-5sin\space x)^2=Acos\space 2x+Bsin\space 2x+C$

$cos^2\space x - 10sin\space x\space cos\space x + 25sin^2\space x= A(cos^2\space x - sin^2\space x)+B(2 sin\space x\space cos\space x)+C$

The form of left hand side doesn't fit in the form of right hand side......

As follows:

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