Explanation:
what is so confusing ? let me know, where you need more explanation.
the angle PTS = 96°
that is the angle between the outer left and the outer right line, so the total angle of everything in the graphic.
now we see 2 sub-angles in there :
PTQ = 28°
RTS = 49°
and we are looking for a third sub-angle QTR.
well, we know, all 3 angles together are the same as the one big angle defined at the beginning.
PTS = PTQ + RTS + QTR
96 = 28 + 49 + QTR
96 - 28 - 49 = QTR
QTR = 19°