So you have

so we must have , , andonlyif .

Okay, have you triedsolvingfor , , and ? If you subtract twice the first equation from the second, you eliminate - . If you subtract the third equation from the first you again eliminate - . Now eliminate by subtracting twice the first of those two equations from the other: .

If it follows that and . What is the only value of A such that does not have to be 0?