calculus help

f(2)  =  12

The inverse to f  =    (x / 3)^(1/2)  = f ^-1

The derivative of this is    (1/3)(1/2)(x/3)^(-1/2)  = (1/6)(x /3)^(-1/2)

So   

df^-1 / dx     at     f(2)   =  12   is

(1/6) (12/3)^(-1/2)  =

(1/6) ( 4)^(-1/2)  =

(1/6) / √4 =

(1 ) / ( 6 * 2)  =

1 /12