Infinitely many solutions means that you have the same thing on both sides of the equation no matter what value of x you plug in, right?
We just need both sides to be 3x then, correct?
If a were equal to 3 and b were equal to 0, we'd have
3x = (3)x + 0
Which is essentially 3x = 3x
So that means a = 3 and b = 0 must work!
Let's say x = 5
3(5) = 3(5) + 0
15 = 15 + 0
15 = 15
That means that a = 3 and b = 0 is your final answer :)