Hello there!
[tex]\begin{bmatrix}6x-9=y\\ y=-3x\end{bmatrix}[/tex]
[tex]\mathrm{Subsititute\:}y=-3x\ \textgreater \ \begin{bmatrix}6x-9=\left(-3x\right)\end{bmatrix}[/tex]
[tex]Isolate\;x\;for\;6x-9=\left(-3x\right)[/tex]
Simply expand (-3x) by removing the parenthesis > -3x
[tex]6x-9=-3x[/tex]
[tex]\mathrm{Add\:}9\mathrm{\:to\:both\:sides} \ \textgreater \ 6x-9+9=-3x+9[/tex]
Simplify!
[tex]6x=-3x+9[/tex]
[tex]\mathrm{Add\:}3x\mathrm{\:to\:both\:sides} \ \textgreater \ 6x+3x=-3x+9+3x[/tex]
Simplify again!
[tex]9x = 9[/tex]
[tex]\mathrm{Divide\:both\:sides\:by\:}9 \ \textgreater \ \frac{9x}{9}=\frac{9}{9} [/tex]
Simplify again!
[tex]x = 1[/tex]
[tex]\mathrm{For\:}y=-3x,\mathrm{subsititute\:}x=1 \ \textgreater \ y=-3\cdot \:1\quad \Rightarrow \quad y=-3[/tex]
Therefore our solutions are...
[tex]x=1,\:y=-3[/tex]
Hope this helps!
Done by substitution as requested.
