[tex]x - y = 12 \\ x = 12 + y \\ \\ x + y = 30 \\ 12 + y + y = 30 \\ 12 + 2y = 30 \\ 2y = 30 - 12 \\ 2y = 18 \\ y = 18 \div 2 \\ \color{red}\boxed{y = 9} \\ \\ x = 12 + y \\ y = 9\\ \\ x = 12 + 9 \\ \color{red}\boxed {x= 21}[/tex]
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