When is $(z_1 - z_2)(z_3 - z_4)$ a negative real number? I know that for this number to be real, then its imaginary part must be 0; however, for the real part, I get
$$ax - aw - cx + cw + by - bz - dy + dz.$$
How do I know when this is negative? I need this in order to solve this problem.