[tex]\it \begin{cases}\it4x+5y=7
\\\;\\ \it y=3x+9 \ \ \ \ (*) \end{cases}[/tex]
[tex]\it 4x+5y=7 \stackrel{(*)}{\Longrightarrow} 4x+5(3x+9) = 7 \Rightarrow 4x+15x+ 45 =7 \Rightarrow
\\\;\\
\Rightarrow 4x+15x=7-45 \Rightarrow 19x= -38 \Rightarrow x = \dfrac{-38}{19} \Rightarrow x= -2[/tex]
[tex]\it x=-2 \stackrel{(*)}{\Longrightarrow} y=3\cdot(-2) +9 \Rightarrow y=-6+9 \Rightarrow y=3
\\\;\\
So,\ we\ have: \begin{cases} \it x=-2
\\\;\\
\it y=3\end{cases}[/tex]
