A radical equation by substitution

Substitution. $t=\sqrt{x}$, so $t\ge0$ and $x=t^2$. The equation becomes
$$t^2-2t-3=0.$$

Roots. $(t-3)(t+1)=0$, so $t=3$ or $t=-1$. Reject $t=-1$ (must have $t\ge0$).

Back. $x=t^2=9$.

Check. $9-2\cdot3-3=0$. Correct.

Conclusion. $x=9$.