A = A
B = 3A + 18
C = A + 42
The sum of the angles of a triangle equals 180°
A + B + C = 180
(A) + (3A + 18) + (A + 42) = 180
5A + 60 = 180
5A = 120
A = 24
B = 3A + 18 = 3(24) + 18 = 72 + 18 = 90
C = A + 42 = C = 24 + 42 = 66
Answer: A=24, B=90, C=66