Salut,
[tex]x-\dfrac{1}x=4\ (\ )^2\Rightarrow\left(x-\dfrac{1}x\right)^2=16\Leftrightarrow x^2-2\cdot x\cdot\dfrac{1}x+\left(\dfrac{1}x\right)^2=16\Leftrightarrow\\\\\Leftrightarrow x^2-2+\dfrac{1}{x^2}=16,\ deci\ \boxed{x^2+\dfrac{1}{x^2}=18.}[/tex]
Green eyes.