Answer
$6~cos^2x-3=3~cos~2x$
Work Step by Step
$sin^2x+cos^2x=1$
$cos^2x-1=-sin^2x$
Use: $cos~2x=cos^2x-sin^2x=2~cos^2x-1$
$6~cos^2x-3=3(2~cos^2x-1)=3~cos~2x$
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