The measure of angle 1 is 75°. Hope you get it.
STR is a triangle as shown in the picture. If Luke decided to subtract 128º from 180º, that's because he understood that the sum of the internal angles in a triangle is 180º, and therefore the angles RST and TRS together make 128º.
As a result, RST will be
128º - TRS
where TRS is the angle at R, as shown in the picture
If anyone has seen the answers to this question then I need please post it
-48 plugging in everything and using PEMDAS it should be -48