The square marker means that the angle is a right angle, or a 90 degree angle.
Rule: For any triangle, the three angles add to 180 degrees.
The angles are: 'a', 'a-30', and 90 degrees
Add them up and set the result equal to 180. Then solve for 'a'
( a ) + ( a - 30 ) + 90 = 180
(a + a) + (-30 + 90) = 180
2a + 60 = 180
2a + 60 - 60 = 180 - 60 ..... subtract 60 from both sides
2a = 120
2a/2 = 120/2
a = 60
Side Note: Since a = 60, this makes a-30 = 60-30 = 30. So the two other angles of the triangle are 60 and 30. Add up the three angles to get 30+60+90 = 90+90 = 180 so that confirms we have the right answer.