If. f(x) = ax² +bx+ c, where a> 0 and D < 0, then we discard it  Solve for x:  $x^4-4\le 0$

Given in-equation is

$\begin{array}{l}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}{x}^4-4\le 0\\ \Rightarrow \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\left(x^2-2\right)\left(x^2+2\right)\le 0\\ \Rightarrow \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\left(x^2-2\right)\le 0\\ \Rightarrow \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}(x+\sqrt{2})(x-\sqrt{2})\le 0\\ \Rightarrow \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}-\sqrt{2}\le x\le \sqrt{2}\\ \Rightarrow \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}\hspace{0.25em}x\in [-\sqrt{2},\sqrt{2}]\end{array}$