What is the final answer according to you?
And isn't the meaning of delta being zero and delta X,Y and Z being zero is infinite solutions ?
When a=1 there are infinite solutions because in that case all delta will have to become zero
When a is not equal to 1 we always have unique solution as delta will be finite for those values.
So, there is no value of b in which there is no solution, either there are infinite solutions or unique solution