Find the linear approximation, for...

$\displaystyle y=mx+b, to f(x) = \sqrt[3]{3x-1}$

at the point x=3

Ok, so

$\displaystyle f(3)=2$

$\displaystyle f'(x)=3x-1$

$\displaystyle f'(3)=8$

now to solve for y=mx+b?..

$\displaystyle 2=8(3)+b$

$\displaystyle b=-22$

$\displaystyle y=8x-22$

this is apparently wrong. What am I doing wrong here?