relative condition number square root of 2

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I need some help with the following task:

I should calculate the term $f_1=(\sqrt{2}-1)^6$ therefore I should approximate $\sqrt{2}$ with $1.4$.

I have some other alternative terms

$f_2=(\sqrt{2}+1)^{-6}$, $f_3=(3-2\sqrt{2})^3$ and more but I think these are enough to understand my problem.

First I showed that these term are equivalent.

Then I should calculte the relative condition number and calculate the error and decide which is the best term to use.

I tried to make functions out of the terms

$f_1(x)=(x-1)^6$ so I changed $\sqrt{2}$ to $x$

And then

$$k_{rel}=\frac{f'(x)*x}{f(x)}$$

My Question is should I plug $\sqrt(2)$ or $1.4$ in? And how do I calculate the error?

Thanks for helping !:)