Pre cal

[input]3^-x+4=71[/input]

3^-x=67
assuming x>0
ln( 3^-x ) = ln(67)
-x*ln(3) = ln(67)
-x=ln(67)/ln(3)
x=-ln(67)/ln(3)

No maybe this will help

3^(-x+4)=71

[input]3^(-x+4)=71[/input]