Question 110901
Start with the given system

{{{3x-y=-15}}}
{{{y=x+1}}}




{{{3x-(x+1)=-15}}}  Plug in {{{y=x+1}}} into the first equation. In other words, replace each {{{y}}} with {{{x+1}}}. Notice we've eliminated the {{{y}}} variables. So we now have a simple equation with one unknown.



{{{3x-x-1=-15}}} Distribute the negative



{{{2x-1=-15}}} Combine like terms on the left side



{{{2x=-15+1}}}Add 1 to both sides



{{{2x=-14}}} Combine like terms on the right side



{{{x=(-14)/(2)}}} Divide both sides by 2 to isolate x




{{{x=-7}}} Divide





Now that we know that {{{x=-7}}}, we can plug this into {{{y=x+1}}} to find {{{y}}}




{{{y=(-7)+1}}} Substitute {{{-7}}} for each {{{x}}}



{{{y=-6}}} Simplify



So our answer is {{{x=-7}}} and {{{y=-6}}}