Explanation:
linear pair of angles are formed at the intersection point of two lines.
the 2 angles are a linear pair, if they are adjacent (next) to each other at the intersection point. that way they cover together a whole side of one of the lines.
the sum of a linear pair is therefore always equal to 180°. a linear pair of angles is also known as supplementary angles (together they represent 180°).
so, bottom line, together they are 180°.
that means
(3a + 5) + (2a - 25) = 180
that we can solve for a :
3a + 5 + 2a - 25 = 180
5a - 20 = 180
5a = 200
a = 200/5 = 40
angle TRS = 3a + 5 = 3×40 + 5 = 120 + 5 = 125°