Question 885604
"<i>Everytime I think I have it, im lost for the next problem, help!</i>"


If that is the case, then try generalizing completely for two linear equations in standard form.


{{{ax+by=c}}} & {{{dx+ey=k}}}.
You wish to use the substitution method.


Solve either equation for one of the variables.
Arbitrarily, x from the first equation:
{{{ax=c-by}}}
{{{x=(c-by)/a}}}
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Substitute for x in the second equation:
{{{d(c-by)/a+ey=k}}}
Solve this for y.
{{{d(c-by)+aey=ak}}}
{{{dc-bdy+aey=ak}}}
{{{-bdy+aey=ak-dc}}}
{{{y(ae-bd)=ak-dc}}}
{{{highlight(y=(ak-dc)/(ae-bd))}}}-----See that this solution for y uses only an expression of constants, so this will be evaluable when we know the values of those constants.
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Again use the previous formula for x, substitute the found formula solution for y, and simplify:
{{{x=(c-by)/a}}}
{{{x=(c-b((ak-dc)/(ae-bd)))/a}}}
{{{x=(c(ae-bd)-b(ak-dc))/(a(ae-bd))}}}
{{{x=(ace-bcd-abk+bcd)/(a^2e-abd)}}}
{{{highlight(x=(ace-abk)/(a^2e-abd))}}}-----again the right-member is an expression of constants.