A particle, attached by a light, inelastic string of length l to a fixed point O, describes with uniform speed a horizontal circile. The string is just capable of supporting n times the weight of the particle without breaking. Prove that the period T of the circular motion cannot be less than

I've worked it out but where it indicates l, i only have h and i have no idea how to change it to l. so basically i just resolved the forces by Newton's second law and then converted the 2 equations into tan x and then equated it.