SOLUTION: Solve the system by substitution. 3x – y = –7 x + y = –9

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Question 81452: Solve the system by substitution.
3x – y = –7
x + y = –9
Answer by mathdoc316(9) About Me  (Show Source):
You can put this solution on YOUR website!
Step 1: Isolate one variable, using one of the equations
Step 2: Substitute that into the other equation to eliminate the variable
Step 3: Solve that resulting equation for the remaining variable
Step 4: Solve for the variable that was eliminated
Step 5: Check the solution
Step 1 will be easy here if we use the 2nd equation, x%2By=-9, to isolate a variable. It could easily be x or y. Let's just do x since it doesn't matter that much.
x+=+-9-y
Step 2: Substitute that expression for x into the first equation
3x+-+y+=+-7
x+=+-9-y
3+%2A+%28-9-y%29+-+y+=+-7
Step 3: Simplify and solve for y
+-27+-+3y+-+y+=+-7
+-27+-+4y+=+-7+
Add 27 to both sides
+-4y+=+20+
Divide both sides by -4
+y+=+-5+
Step 4: Find x
+x+=+-9+-+y
+x+=+-9+-+%28-5%29+=+-4+
Step 5: Check the solution
+x+=+-4, y+=+-5+
+3%2Ax+-+y+=+-7+
+x+%2B+y+=+-9+
+3+%2A+-4+-+%28-5%29+=+-12+%2B+5+=+-7+ YES
x+%2B+y+=+-4+%2B+%28-5%29+=+-9+ YES
The solution is correct