SOLUTION: Find the solution to system of equations by substitution. Problem: Y=2X+7 and Y=-X-5. I am confused on which e

Question 121375: Find the solution to system of equations by substitution.

Problem: Y=2X+7 and Y=-X-5. I am confused on which equation do i insert and when. For this problem I tried Y=-X(2X+7)-5, Then I'm stumped, I think the -x multiplied by the 2x is messing me up. Plus both of these start with Y=. The other examples problems show x+2y=6 and 4y=12-2x ? Totally different. Can you help me sort out how to do substition? How to start. I have went thru example after example in book and I keep getting the wrong answers I am missing a step or concept i think.
Answer by checkley71(8403) (Show Source):
You can put this solution on YOUR website!
Y=2X+7 NOW REPLACE THE Y IN THE OTHER EQUATION WITH(2X+7) & SOLVE FOR X.
2X+7=-X-5
2X+X=-5-7
3X=-12
X=-12/3
X=-4 ANSWER.
Y=2*-4+7
Y=-8+7
Y=-1 ANSWER.
PROOF
-1=-(-4)-5
-1=4-5
-1=-1
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X+2Y=6 OR X=6-2Y
NOW REPLACE X IN THE OTHER EQUATION WITH(6-2Y) & SOLVE FOR Y.
4Y=12-2(6-2Y)
4Y=12-12-2Y
4Y+2Y=0
2Y=0
Y=0 ANSWER.
X+2*0=6
X=6 ANSWER.
PROOF
4*0=12-2*6
0=12-12
0=0