The upper part is the question and lower part is my answer.
Is a=-1, b=3, c=1 acceptable?
By finding the notation ( ) in the text of your question, it is certain that your exercice concern the set of real numbers (not complex numbers) .
The equality makes sence if and only if the variables a and c are defined as positive real numbers (includes the possible value of zero).
So finding c=-1 makes the value of c out of defined range and thus there is no need to continue calculating the values of a and b.