Question 20 in section 1.2 is quite intuitive to see that it is true. However, I am having difficulties starting to prove what agrees with my intuition.

Currently, I am trying to use the argument to show that $tz_1+(1-t)z_2$ is indeed the line joining $z_1$ and $z_2$.

Does anyone have any hints as to how to start proving Question 20?

Thank you