If PST is a straight line and PQ = QS = SR in the diagram,…

Explanation

∠PSQ = ∠SPQ = 24 ( base ∠s of iso △)

∠SQR = <PSQ + <SPQ [ 1 ext < = 2 int.  <s]

QSR = 180 - 2(48) = 84º ( vertical < of an iso △)

Now at S: y + 24º + 84º = 180 ( sum of <s on a straight line)

y = 180 - 108º = 72º