Thread: Solving Systems of Equations by Substitution Question

Hi all! I'm trying to help my boyfriends son with his math homework and although I remember doing it, I'm having a hard time recalling how exactly to do it. Can someone explain to me the rules with solving systems of equations by substitution? Here is an example of what his worksheet looks like.

y = x + 2
y = -3x

Simple right? I keep getting -8 = 2 + 2 or -6 = 2 + 2. What am I doing wrong? I must be missing a step or doing something wrong with a number. The negative throws me off with this as well. The x is considered negative with the 3 right? So when I subtract x from both sides its making -3x - x, making it -4x?

I know this is simple but I just need a little bit of tutoring myself before I can help him! Thanks for any answers!!!

Originally Posted by Jess51483

y = x + 2
y = -3x

Substitute -3x for y:
-3x = x + 2
subtract x from each side:
-4x = 2
divide by -4:
x = 2/-4 = -1/2