$\begin{array}{l}
{d_1}:\,\,\,y = x + 3;\,\,\,{d_2}:\,\,\,y = 3x + 7\\
b)\,\,\,{d_1} \cap Oy = \left\{ A \right\} \Rightarrow A\left( {0;\,\,3} \right)\\
{d_2} \cap Oy = \left\{ B \right\} \Rightarrow B\left( {0;\,\,7} \right)\\
\Rightarrow Trung\,\,diem\,\,\,I\,\,\,cua\,\,AB\,\,\,\,la:\,\,\,\,I\left( {0;\,\,\frac{{3 + 7}}{2}} \right) \Rightarrow I\left( {0;\,\,5} \right).
\end{array}$