Hello ryogaki.

Here's what I think the diagram looks like:

You do not need to use the sine rule because it is a 45 degree triangle. Since a length is tangent to the circle and another goes through the centre, we know that it is a right angle triangle, and so we can use the pythagorus theorum.

Length CD = AD because it is a 45 degree triangle.

Length BC also equals CD and AD because it is the radius of the same circle.

Let BC = CD = AD = x

Now using pythagorus theorum:

We know that length AC = (x + 15)

Therefore the equation becomes:

Now can you carry on from there?