You "trialled" x=8 and it worked.
You did not show us that you had "trialled" any other number .
We are not here to do all your homework1
1) $$(\sqrt a)^{10}=a^5$$
Now that is a better question for Google.
I cannot see the diagrams / pictures either.
Hint ^ means power of
Now jsut do it on the calc on the home page.
Start by multiplying both sides by 3 that will get rid of the fractions.
$$a^{-12}=\frac{1}{a^{12}}$$
$$\\f(x)=\frac{f(x)-\sqrt x}{5}\\\\ 5f(x)=f(x)-\sqrt x\\\\ 4f(x)=-\sqrt x\\\\ f(x)=\dfrac{-x^{0.5}}{4}\\\\ f'(x)=\dfrac{-0.5x^{-0.5}}{4}\\\\ f'(x)=\dfrac{-0.5}{4\sqrt{x}}\\\\ f'(x)=\dfrac{-1}{8\sqrt{x}}\\\\$$
I think that is what you wanted.
Take it all to one side and put equal to 0
Then use the quadratic formula.
Think of a number line.
start at -5 then move 2/7 of a unit further to the negative end. Where do you end up?
+118732 
