Since PT is parallel to QR, according to the Corresponding Angles Theorem ∠SPT is congruent to ∠SQR, and ∠STP is congruent to ∠SRQ. Therefore, by the Angle-Angle Similarity Theorem △ QRS is similar to △ PTS. Let's write a proportion using the expressions for the lengths of the sides of both triangles.
SP/SQ=PT/QR
⇕
4/SQ=6/12
Finally, we will solve this equation for SQ.