Incomplete quadratic equations

Since there is only one term with $x$, we can isolate the unknown directly: $$3x^2-12=0$$ $$3x^2=12$$ $$x^2=\dfrac{12}{3}=4$$ $$x=\pm\sqrt{4}=\pm2$$ $$x_1=2 \ \text{and} \ x_2=-2$$

Since there is no independent term, we can take the common factor of the $x$: $$x^2-x=0$$ $$x(x-1)=0$$ $$x_1=0; \ \text{and} \ x-1=0\rightarrow x_2=1$$

Since there is only one term with $x$, we can isolate the unknown directly: $$1-4x^2=-8$$ $$1+8=4x^2$$ $$\dfrac{9}{4}=x^2$$ $$x=\dfrac{\pm\sqrt{9}}{\pm\sqrt{4}}=\dfrac{\pm3}{\pm2}$$ $$x_1=\dfrac{3}{2} \ \text{and} \ x_2=\dfrac{-3}{2}$$