??Absolutely not sure what the question is. What should X be? Ther term for x from the first example??
As Alan pointed out, the second system will not have a unique solution. If it has no solution at all or an infinite number of solutions depends on the rank of the extended matrix. If its 3, we will have no solution, if its 2 there is an infinite number of solutions. The latter case is for X=(sqrt(2)-1)*(a+b).
Mathcads result is inaccurate and misleading, but MC can do better using "fully":
