tanx-(1/tanx)
=(tan^2x-1)/tanx
=-4
tanx/(1-tan^2x)=1/4
2tanx/(1-tan^2x)=1/2
tan2x=1/2
tan(π/4-2x)
=(1-tan2x)/(1+tan2x)
=(1/2)/(3/2)
=1/3