You solve for one of the variables.
3x-9y+3y=17 combine like terms on left side (3y-9y)
3x-6y=17 subtract 3x from both sides
-6y=17-3x divide both sides by -6
y=(3x-17)/6
Since this is just a line, there are infinitely many solutions. For every real value of x there will also be one real unique corresponding y value. Or in other words there will by infinitely many unique (x,y) solutions to the original expression that you started with.
