Explanation:
always remember (for your life) : the sum of all angles in a triangle is always 180°.
and the sum of all angles around a single point on one side of a line is also always 180°.
so,
180 = 65 + 51 + angle PSR
angle PSR = 180 - 65 - 51 = 64°
angle PST is the supplementary angle to angle PSR (that means together they have 180°), because of the principle of all angles around a single point (S) on one side of a line (TSR).
so,
180 = angle PST + angle PSR = angle PST + 64
angle PST = 180 - 64 = 116°