$\text{a)}$  

$2x^2-3=6-x^2\mid -6+x^2$  

$3x^2-9=0\mid :3$  

$x^2-3=0$  

$\left(x-\sqrt{3}\right)\left(x+\sqrt{3}\right)=0$  

$x-\sqrt{3}=0\text{lub}x+\sqrt{3}=0$  

$x=\sqrt{3}\text{lub}x=-\sqrt{3}$  

$\text{b)}$