In order to solve this system by substitution, we just need to apply the value of x from the second equation in the first one. So we have that:
[tex]\begin{gathered} x=8y+9 \\ \\ -3x+6y=45 \\ -3(8y+9)+6y=45 \\ -24y-27+6y=45 \\ -18y=45+27 \\ -18y=72 \\ 18y=-72 \\ y=\frac{-72}{18}=-4 \end{gathered}[/tex]Now that we have the value of y, we can find the value of x:
[tex]x=8y+9=8\cdot(-4)+9=-32+9=-23[/tex]So we have that x = -23 and y = -4
