Using "a" to represent the measure of angle a, along with the fact that the sum of angles in a triangle is 180°, the problem statement tells us
... (3a -20) + (a -15) + a = 180
... 5a - 35 = 180 . . . . . . . . . . . eliminate parentheses, collect terms
... a - 7 = 36 . . . . . . . . . . . . . . divide by the coefficient of "a"
... a = 43 . . . . . . . . . . . . . . . . . add 7
... b = a - 15 = 28 . . . . . . . . . . use the relationship of a and b
... c = 3a - 20 = 3·43 -20 = 109 . . . . . use the relationship of a and c
Check our work
... a + b + c = 43 + 28 + 109 = 180 . . . . the sum is 180, as it should be
The measures of the three angles, a, b, c, are 43°, 28°, and 109°, respectively.