Change x-y=9k to x=y+9k.
Substitute these two forms of x into 2x+3y=6 respectively.
Equation set 2(5k-y)+3y=6 ... gives 1.
2(y+9k)+3y=6...2 formula
1 is simplified to 10k+y=6.
Equation 2 is simplified to 5y+ 18k=6.
Lianlide 6- 10k=(6- 18k)/5
The solution is k=3/4.