If you mean 3 + 2x - y = 0 then:
2x - y = -3
10x + 7y = -3
multiply the top equation by 7 on both sides
14x - 7y = -21
10x + 7y = -3
add the bottom equation to the top
24x + 0y = -24
10x + 7y = -3
divide the top by 24 on both sides
x + 0y = -1
10x + 7y = -3
Now we know x = -1
multiply the top equation by -10
-10x + 0y = 10
10x + 7y = -3
add the top equation to the bottom
-10x + 0y = 10
0x + 7y = 7
divide the bottom equation by 7 on both sides
-10x + 0y = 10
0x + y = 1
Now we know y = 1
Answer: x = -1, y = 1