Hello fardeen_genFirst, use the Sine Rule to find angle B:

and

Then, the Sine Rule again to find AB:

, noting that

And if the perpendicular distance from B to AC produced = , then

Then, we assume that

- the picture is on the point of slipping at B
- the picture has mass m, and is uniform, so the centre of mass is at the mid-point of BC
- the forces at B are
- the tension in the string is

Resolve vertically:

Resolve horizontally:

Take moments about B:

You now have all you need to eliminate and and obtain an equation for .

Grandad