If the lines are perpendicular to each other, then there slopes would be opposite and reciprocal to each other.
In Justin's case, slope of first line is -1/2, so it would be 2 in second, but it's -2 there as a slope of 2nd line. so, he's wrong.
Now, in Robert's case, reciprocal of -1/2 would be 2, so he's correct
In short, "Robert is Correct"
Hope this helps!
