The sum of the angles of a triangle add to 180.
Supplementary angles are two angles which add to 180.
If the ratio of the angles is 3:4:8, that means there are 15 parts (3+4+8).
Angle 1: 180/15*3=36
Angle 2: 180/15*4=48
Angle 3: 180/15*8=96
36+48+96=180 ✅
The largest angle is 96 so the supplement is 180-96=84.