Exercise 17

Show that the line segments $\overline{(a_{1},a_{2})(b_{1},b_{2})}$ and $\overline{(c_{1},c_{2})(d_{1},d_{2})}$ have the same lengths and slopes if $b_{1}-a_{1}=d_{1}-c_{1}$ and $b_{2}-a_{2}=d_{2}-c_{2}$. Is that only if?