Systems of equations: solve each system by setting the equations equal to each other and solving.
y=x
y= -x

Since ' x ' and '-x ' are both equal to ' y ', they must be equal to each other.

                                     x = -x

Add ' x ' to each side:     2x = 0

Divide each side by  2 :    x = 0
                        whence   y = 0 .

That's the only possible solution to the system [ y = x  and  y = -x ] .

I’d say letter a..