Question 173215
Note: I'm assuming that you left out the variable "y" in the second equation and it should look like {{{x-7y=4}}}







Start with the given system of equations:


{{{system(x+y=12,x-7y=4)}}}



Let's solve this system by use of substitution




Now in order to solve this system by using substitution, we need to solve (or isolate) one variable. I'm going to solve for y.





So let's isolate y in the first equation


{{{x+y=12}}} Start with the first equation



{{{y=12-x}}}  Subtract {{{x}}} from both sides



{{{y=-x+12}}} Rearrange the equation



---------------------


Since {{{y=-x+12}}}, we can now replace each {{{y}}} in the second equation with {{{-x+12}}} to solve for {{{x}}}




{{{x-7highlight((-x+12))=4}}} Plug in {{{y=-x+12}}} into the second equation. In other words, replace each {{{y}}} with {{{-x+12}}}. Notice we've eliminated the {{{y}}} variables. So we now have a simple equation with one unknown.




{{{x+(-7)(-1)x+(-7)(12)=4}}} Distribute {{{-7}}} to {{{-x+12}}}



{{{x+7x-84=4}}} Multiply



{{{8x-84=4}}} Combine like terms on the left side



{{{8x=4+84}}}Add 84 to both sides



{{{8x=88}}} Combine like terms on the right side



{{{x=(88)/(8)}}} Divide both sides by 8 to isolate x




{{{x=11}}} Divide






-----------------First Answer------------------------------



So the first part of our answer is: {{{x=11}}}










Since we know that {{{x=11}}} we can plug it into the equation {{{y=-x+12}}} (remember we previously solved for {{{y}}} in the first equation).




{{{y=-x+12}}} Start with the equation where {{{y}}} was previously isolated.



{{{y=-(11)+12}}} Plug in {{{x=11}}}



{{{y=-11+12}}} Multiply



{{{y=1}}} Combine like terms 




-----------------Second Answer------------------------------



So the second part of our answer is: {{{y=1}}}










-----------------Summary------------------------------



So the solutions are:



{{{x=11}}} and {{{y=1}}}



which form the ordered pair *[Tex \LARGE \left(11,1\right)]