JEE Main 2022 · 27 Jun · Shift 2, Physics question 35

A stone tide to a string of length L is whirled in a vertical circle with the other end of the string at the centre. At a certain instant of time, the stone is at its lowest position and has a speed $u$. The magnitude of change in its velocity, as it reaches a position where the string is horizontal, is $\sqrt{x\left(u^2-gL\right)}$. The value of $x$ is-