Answer:
0
Step-by-step explanation:
a + b = a - b
this can only be true, if one of these two numbers is 0 (in this case b, as b = -b, which is only true for b = 0).
the other one then can be of any value, as any number + 0 is equal to the same any number - 0.
now, as one of the 2 numbers has to be 0, the product of these two numbers has to be 0.
