System solving through elimination

Question

3x-2y= -1

3x-4y= 9

 

by adding, I get:

 

-6y=8

/6=/6

y=4/3 ?

 

Do I need to multiply bottom equation by -1, before adding?

3x-2y=-1

-3x+4y= -9

2y=-10

/2=/2

y=-5

 

Answer 1

If you add the two equations, you get:

 

3x-2y=-1

3x-4y= 9

----------

6x-6y=8

 

So, yes, you need to multiply the second equation by -1 first then add to eliminate the x term:

 

  3x-2y=-1

-3x+4y=-9

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

  0+ 2y=-10

 

y= -5

 

Solve for x:

 

3x-2(-5)=-1

3x+10=-1

3x=-11

 

x=-11/3

 

Check by plugging x= -11/3 and y= -5 into both of the original equations.