Question 347200
Some say a some say b.
Are you taking a poll? 
There is only 1 right answer.
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You can do it two ways. 
Plug the values in and see which one satisfies the equation. 
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Although that works, it won't help you next time you don't have an answer to choose from.
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You can solve by elimination or substitution.
Substitution: Use 1 equation to solve for 1 variable then substitute for that variable in the second equation.
1.{{{3x + y = 1}}}
2.{{{-x + 2y = 9}}}
From eq. 2,
{{{x=2y-9}}}
Substitute into eq. 1,
{{{3(2y-9)+y=1}}}
{{{6y-27+y=1}}}
{{{7y=28}}}
{{{highlight(y=4)}}}
Then from eq. 2,
{{{x=2(4)-9}}}
{{{x=8-9}}}
{{{highlight(x=-1)}}}
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The other method is elimination.
Manipulate the equations to eliminate one of the variables.
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1.{{{3x + y = 1}}}
2.{{{-x + 2y = 9}}}
Multiply eq. 2 by {{{3}}} and add the equations together.
{{{3x+y-3x+6y=1+27}}}
{{{7y=28}}}
{{{y=4}}}
Then use either equation to solve for {{{x}}}.
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Isn't that better than guessing?