Question 765455
{{{f(x)=4x}}} first find {{{f^-1}}}

{{{y=4x}}}..switch {{{x}}} and {{{y}}}

{{{x=4y}}}.....solve for {{{y}}}

{{{x/4=y}}}

so, 

{{{f^-1 (x) = x/4}}}

now find  {{{f(f^-1(-6))}}}

{{{f^-1 (-6) = 4(-6/4)}}}

{{{f^-1 (-6)=-6}}}