Answer:
a = -3, b = 6
Explanation:
ax + 6 = -3x + b
If a = -3 and b = 6, then the equation would be -3x + 6 = -3x + 6, which would have infinitely many solutions.
If we isolate x, we have (a + 3) x = b - 6. We want the equation to be 0 = 0 for infinitely many solutions, so a has to be -3 and b has to be 6.