Question 102508
{{{-15x+3y=9}}}
{{{y=3+5x}}}
Subtitution means to plug the value of y from the second equation into the first equation.
{{{-15x+3(3+5x)=9}}}
{{{-15x+9+15x=9}}}
Collecting the x's, we end up the tautology:
{{{9=9}}}, which is not much help to us.
So, returning to the original equations:
{{{-15x+3y=9}}}
{{{y=3+5x}}}
Subtracing 5x from both sides of the second equation, we have:
{{{-5x + y = 3}}}.
Aligning the two linear equations:
{{{-15x+3y=9}}}
{{{-5x+y=3}}}
Dividing the first equation by 3, we have:
{{{-5x+y=3}}}
So, we see that we really do not have two equations. We have just one equation. The first and second are just different expressions of the same thing.