You introduced t in the second line without explaining it's relation to .
It would be better if you could explain how they are related
where O, the opposite, is
where A, the adjacent, is
and H, the hypotenuse, is
I missed out the explanation on this (which I think is best shown when the triangle is actually drawn out) but will keep it in mind whenever I do this type of question in the future.
Otherwise, would you believe that my working is correct?