Since this is a pieciwise function we need to determine which expression to use for the value of x given. For the definition of the function we notice that for every x grater than 4 we need to use the function:
![-2x-6](https://img.qammunity.org/2023/formulas/mathematics/college/844r9407j9qznsbca3xus00xfbcz5vijey.png)
Since in this case we want to evaluate the function at x=6 and this value is greater that four we need to use this expression.
Then we have that:
![\begin{gathered} f(6)=-2(6)-6 \\ =-12-6 \\ =-18 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/4mwfpmgnam3c2r1rq46i2jg1jqa0dnxox6.png)
Therefore the value of the function at x=6 is -18