Question 110798
Start with the given system

{{{x+y=12}}}
{{{y=2x}}}




{{{x+(2x)=12}}}  Plug in {{{y=2x}}} into the first equation. In other words, replace each {{{y}}} with {{{2x}}}. Notice we've eliminated the {{{y}}} variables. So we now have a simple equation with one unknown.




{{{3x=12}}} Combine like terms on the left side



{{{x=4}}} Divide both sides by 3 to isolate x





Now that we know that {{{x=4}}}, we can plug this into {{{y=2x}}} to find {{{y}}}




{{{y=2(4)}}} Substitute {{{4}}} for each {{{x}}}



{{{y=8}}} Simplify



So our answer is {{{x=4}}} and {{{y=8}}}