Answer: Last choice, aka a^2-3(3+7)
Explanation:
Tangent-secant Theorem: If a tangent & secant are drawn from the same exterior point, the tangent's length squared equals the length of the product of the external part of that secant and the full secant.
Here, the smaller segment is the tangent. Its length is a.
The larger segment is the secant. The length of its external part is 3. Its full length is 7+3.
Return to the Tangent-secant Theorem:
Tangent^2=(External part of Secant)(Secant)
Substitute our values:
a^2=3(3+7)
Answer is the last choice.