Forces and equilibrium — practice question

A smooth ring $R$ of mass $m\ \text{kg}$ is attached to a light inextensible string $ARB$. The string’s ends are fixed at points $A$ and $B$, with $A$ directly above $B$. The string is taut, and angle $ARB = 90^{\circ}$. The angle between the segment $AR$ of the string and the vertical is $45^{\circ}$. The ring is kept in equilibrium in this arrangement by a force of magnitude $2.5\,\text{N}$, acting on the ring in the direction $BR$ (see diagram).