​ help? I don't know how to do this question

Help? I don't know how to do this question

Tangent Secant Theorem

$\overline{AD}^2 = \overline{AB} \cdot (\overline{AB} + \overline{BC}) \\ \overline{AD}^2 = 16 \cdot (16 +9) \\ \overline{AD}^2 = 16 \cdot 25 \\ \overline{AD} = 4 \cdot 5 \\ \overline{AD} = 20$