SOLUTION: Can you show me how to solve this system of equations: y=3x x+21=-2y by the substitution method?

Algebra ->  Coordinate Systems and Linear Equations -> SOLUTION: Can you show me how to solve this system of equations: y=3x x+21=-2y by the substitution method?      Log On


   

Question 119807: Can you show me how to solve this system of equations: y=3x
x+21=-2y
by the substitution method?
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
Start with the given system
x%2B21=-2y
y=3x



x%2B21=-2%283x%29 Plug in y=3x into the first equation. In other words, replace each y with 3x. Notice we've eliminated the y variables. So we now have a simple equation with one unknown.


x%2B21=-6x Multiply


x=-6x-21Subtract 21 from both sides


x%2B6x=-21 Add 6x to both sides


7x=-21 Combine like terms on the left side


x=%28-21%29%2F%287%29 Divide both sides by 7 to isolate x



x=-3 Divide




Now that we know that x=-3, we can plug this into y=3x to find y



y=3%28-3%29 Substitute -3 for each x


y=-9 Simplify


So our answer is x=-3 and y=-9