The last one would be negative.
Positive times positive = positive
Positive times negative = negative
Negative times negative = positive
So basically, if there is an odd number of negatives, then it'll end up negative, with some exceptions.
So now we know it has to either be the first one or the last one.
The first one is multiplied by 0, so it can't be that one.
So it has to be the last one.