have look a at diagram above
=> $O$ is the starting point => $0<\alpha<\beta<\pi/2$ => $OL=OB=d$
i worked the distance $LB$ to be $2d\sin((\beta-\alpha)/2)$ but, i am struggling to understand this 'The distance between $L$ and $B$ when $L$ has walked a distance $d$ must be less than $kd$, where $k$ is some constant. Find $k$.
How do i find $k$ ?