Question 102597
Substitution means plugging one equation into the other.

So if we plug y = 2x into the first equation we get
{{{x + 2x = 12}}}
Combine like terms
{{{3x = 12}}}
Divide both sides by 3
{{{x = 4}}}
Then we can plug in that result to the second equation.
{{{y = 2x}}}
{{{y = 2 * 4}}}
{{{y = 8}}}
We can check this by plugging it back into the first equation.
{{{x+y=12}}}
{{{4+8=12}}}
12=12
Checks out.
That's it for this problem.