Help on this math problem

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<p>Angle PRQ is a right angle because RQ is a tangent line. PR is 6, and PQ is 6 + 1 = 7. By the Pythagorean theorem, RQ is squareroot of 13. SQ is 1, so RS is (squareroot of 13) - 1.</p>

<p>Sorry for no symbols. Where does the problem come from? It is not a particularly hard one.</p>