I began with the assumption that the shortest time would be obtained in such a way that the cop passes the point at which the burglar leaves the shore, and then some time after that, begins to swim in order to intercept the burglar. So I stated:

is the time between the cop reaching the point where the burglar left the shore (B) and the point at which he begins to swim (A).

is the time the cop takes after he begins to swim before he catches the burglar.

It takes the cop 10 seconds to reach B. Using the law of cosines,

.

Does that make sense or did I lose you?