Rozwiążmy podane układy równań metodą podstawiania:

$a\text{)}\{\begin{array}{l}x=2y\\ x+4y=12\end{array}$  

$\{\begin{array}{l}x=2y\\ 2y+4y=12\end{array}$      

$\{\begin{array}{l}x=2y\\ 6y=12\end{array}$  

$\{\begin{array}{l}x=2y\\ y=2\end{array}$  

$\{\begin{array}{l}x=2\cdot 2\\ y=2\end{array}$  

$\{\begin{array}{l}x=4\\ y=2\end{array}$  

$b\text{)}\{\begin{array}{l}y-3x=2\\ 2x-5y=3\end{array}$  

$\{\begin{array}{l}y=2+3x\\ 2x-5\cdot \left(2+3x\right)=3\end{array}$  

$\{\begin{array}{l}y=2+3x\\ 2x-10-15x=3\end{array}$  

$\{\begin{array}{l}y=2+3x\\ -13x=13\end{array}$  

$\{\begin{array}{l}y=2+3x\\ x=-1\end{array}$  

$\{\begin{array}{l}y=2+3\cdot \left(-1\right)\\ x=-1\end{array}$   

$\{\begin{array}{l}y=2-3\\ x=-1\end{array}$  

$\{\begin{array}{l}y=-1\\ x=-1\end{array}$