Question

When using the substitution method, what expression for x in the first equation would be substituted in the second equation?

x + 3y = -7
2x + 4y = -4

Answer 1

Okay, so basically, in the first equation, get x alone so that it's in the form:
x = stuff

So what's the first step in doing that?

Answer 2

would i subtract it from both sides?

Answer 3

Okay, so what did you get?

Answer 4

Am I getting rid of the x or the y? If i did x it would it be 3y = -7 - x ?? Im not sure how to do this..

Answer 5

or 3y = -6?

Answer 6

You want it to be x = stuff

Answer 7

x + 3y = -7
So we're just going to subtract the 3y from the left side so that x is alone.
x+3y-3y = -7-3y
x=-7-3y

So in the second equation, everywhere you see and x, put -7-3y instead.

Answer 8

after you wrote that I realized i was supposed to subtract the 3y, I just get confused on that part.. Thank you!

Answer 9

2nd equation: 2x + 4y = -4
So if you were solving it, you would find the x and get rid of it, in its place put -7-3y
2(-7-3y) + 4y = -4
Distribute the two:
-14 - 6y + 4y = -4
Add -6y to 4y
-14 - 2y = -4
Work on getting y alone, so we'll add 14 to that side to get rid of the -14 (and add it to the right side of the equation so it's still equal.)

-14+14 - 2y = -4+14
-2y=10
divide off the -2
(-2y)/-2=10/-2
y = -5

So now we know y is equal to -5. So in either of the original two equations (whichever one you dislike the least), substitute -5 for y and solve it so you can find out what x really is. I'll use the first equation for this:

x + 3y = -7
x + 3(-5) = -7
x + -15 = -7
x + -15 + 15 = -7 + 15
x= 8


So in your system of equations, y = -5 and x = 8