Ruby's hours will represent x+6
Svetlana's hours will represent 4(x+6)
Issac will represent x. 
The equation would read:
x+(x+6)+(4(x+6))=126
x+x+6+4x+24=126
6x+30=126
Subtract 30 from both sides. 
6x=96
Divide 6 on both sides to get x by itself.
x=16
So:
Issac worked 16 hours
Ruby worked 22 hours
Svetlana worked 88 hours. 
To check your answer, plug in the values:
16+22+88=126
126=126
Hope this helps.