You ought to take into concideration such basic notions as sine and cosine lines. Since the right triangle is given with C obviously being the right angle and B and C - acute angles (their sum is 90), let the triangle be formed of hypotenuse=the radius of the trigonometric circle in a rectangular coordinate system = 1 and the sides AC and BC being cathetuses. If to assume that the cathetus AC is lying on Ox axis, then AC is the cosine line of A, BC is the sine line of A and the right triangle ABC stands vertically. If to rotate it so that BC became the horizontal cathetus and AC vertical one, then BC would be the cosine line of B, AC - the sine line of B. So, it seems that sin A = cos B = 5/13 that is the answer to your question.
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