Notice that the subtraction of two complex numbers (z1- z2) implies the use of the opposite for the real and imaginary part of the complex number that is subtracted (in our case of z2). When we do such, the complex number z2 gets reflected about the origin (0,0), and then the real components of the two numbers get added among themselves and the imaginary components get added among themselves.
The diagram that shows such reflection about the origin [ z2 = 3 + 5 i being converted into -3 - 5 i] and then the combination of real parts [-3 + 5 = 2] and imaginary parts [-5 i - 3 i = - 8 i], is the very first diagram shown.
