Solve the following system, where a and b are constants

2x+y=a
3x+6y=b

Solve the first to get
y=a-2x

Substitue into the second to get
3x+6(a-2x)=b

Solve for x
3x+6a-12x=b
x=-(b-6a)/9

Substitute into the first
2(-(b-6a)/9)+y=b

Solve for y
y=2(b-6a)/9