we have the expression
[tex]\begin{gathered} 14cos^2x-7 \\ 7(2cos^2x-1) \end{gathered}[/tex]
Remember that
[tex]cos(2x)=2cos^2x-1[/tex]
substitute in the given expression
[tex]\begin{gathered} 7(2cos^{2}x-1) \\ 7(cos(2x)) \\ 7cos(2x) \end{gathered}[/tex]
