Question

{ 3x + y = 5
{ 2x+y =4

Answer 1

3x + y = 5
2x + y = 4

Subtraia as equações:

x = 1

Logo 

3x + y = 5
3 + y = 5
y = 2

Answer 2

{3x+y=5
{2x+y=4
y=5-3x
2x+(5-3x)=4
2x+5-3x=4
2x-3x=4-5
-x=-1.(-1)
x=1
y=5-3x
y=5-3.1
y=5-3
y=2
S={(1, 2)}