Square Both Sides

A square undoes a square root: ${\left(\sqrt{9}\right)}^{2}$ is 9, because $\sqrt{9}$ is by definition the number whose square is 9. So when x is stuck under a root, square both sides of the equation and the root is gone.

Take $\sqrt{x-3} = 5$. Squaring the left leaves $x-3$, and squaring the right gives 25. So $x-3 = 25$, and x is 28. Check it in the original: $\sqrt{28-3}$ is $\sqrt{25}$, which is 5.

On the board the move is Square both sides. Squaring can bring in an answer that does not work (you will meet one two lessons from now), so afterwards the board asks you to Check the answer in the original equation before it counts. A root also tells you something at a glance: $\sqrt{x} = -2$ has no solution at all, because a square root is never negative.