Two linear equations can have zero, one, or infinitely many solutions. Zero if the are parallel, have the same slope, and have different y-intercepts, one if they they have different slopes, infinite if they have the same slope and y-intercept (as they both describe the same line :P)
In this case if you factor the right side by three, you have the same exact expression as the left side. This means that each side represent the same line and has infinitely many solutions.