Question 70506
y=x
y=-x

Let's take your problem #1 as an example.

First, you need to set both equations = 0.
y = x becomes y - x = 0
y = -x becomes y + x = 0

Second, perform subtraction between those equations.
y - x = 0
y + x = 0
__________ -
0 - 2x = 0

Third, simplify.
0 - 2x = 0
-2x = 0

Fourth, divide both sides by -2.
-2x/-2 = 0/-2
x = 0

Fifth, since we know that x = 0, recall that the original problem was:
y=x
y=-x
Therefore, we can take either y=x or y=-x to substitute x as a 0.
Either way, y = 0.
So, the answer is x = 0 and y = 0.

Why don't you try question #2 your own? If you have problems, post back the question and I'll be glad to help you :)