In the diagram below we have QR is parallel to ST, PQ=8, QS=6, and PT=10.5
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PQR and PST are similar. PQ + QS = 14, and PQ = 8, so PQ is 8/14, or 4/7, the length of PS.
Same proportion for PT. PT is 10.5, and PR is 4/7 of that length. Can you find out from here?