the angle of elevation of the top of building 24m high is obsessed from the top of and from the bottom of a vertical ladder and found to be 45°and 60° respectively. find the height of the ladder
"obsessed"? I think you mean observed.
As always, draw a diagram, and review your basic trig functions.
You can see how to find the height h of the ladder:
24/tan60° = (24-h)/tan45°