Ok... not sure if I'm allowed to help since I'm new... but here goes...

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.