we know that
If a tangent segment and a secant segment are drawn to a circle from an exterior point, then the square of the measure of the tangent segment is equal to the product of the measures of the secant segment and its external secant segment
so
AB²=BD*BC
BD=12
BC=12+15----> BC=27
AB²=12*27----> 324
AB=√324----> AB=18
the answer is
AB=18 units