Ideal simple pendulum.

Answer

3

Explanation

Since, the pendulum started with no kinetic energy. Conservation of energy implies that the potential energy at extreme position must be equal to the original potential energy, so the vertical will be same at other sxtreme position,
so we use
`implies L cos alpha =l+(L-l) cos theta`
`implies cos theta=(L cos alpha -l)/(L-l)`
`implies theta =cos^(-1) [(L cos alpha-l)/(L-l)]`