Theorem required: secants theorem
All secants drawn from a point exterior to a circle will have the products of the secant and the external segment equal.
For the given example, we have, from the secants theorem, CA*CB=CE*CD,
where CA and CE are secants, and CB and CD are external segments.
Substitute numbers,
CA*CB=CE*CD
=>
CA*6 = 12*5
=>
CA = 12*5/6=10 => AC=10