To express a system/simultaneous equations use aligned.
\begin{aligned} 2x + 3y &= 4 \\ y &= 2x - 5 \end{aligned}
Notice that the & here is used to create a vertical alignment (stacking the "=" symbols) by the character following the &. So, &= in each line produces the equation you see above.
You can also use {\color{a color} expression} to change the color. See a list of available colors.
\begin{aligned} 2x + 3y &= 4 \\ y &= {\color{orange}2x - 5} \\ 2x + 3\left({\color{orange} 2x - 5}\right) &= 4 \\ 2x + 6x - 15 &= 4 \\8x - 15 &= 4 \\ 8x &= 19 \\ x &= {\color{red}\dfrac{19}{4}} \\ y &= 2\left({\color{red}\dfrac{19}{4}}\right) - 5 \\ &= \dfrac{38}{4} - \dfrac{20}{4} \\ &=\dfrac{18}{4} = {\color{orange}\dfrac{9}{2}} \\[1pc]\end{aligned}
So we can say that the solution in (x, y) form is \left({\color{red}\dfrac{19}{4}}, {\color{orange} \dfrac{9}{2}}\right)