In order for there to be infinitely many solutions to the equations, that means that all three of those equations are a multiple of each other. You can think of this as a system of equations and treat "a" as a constant, and try to solve either by addition or substitution. I would suggest multiplying the second equation by (-2). This will allow you to add this equation to the third equation and a variable will cancel out, and you also get a nice little constant on the right hand side that is very useful. Try that and see if that helps you progress a little.