Answer :
x + y = 4
y = x + 4
x + (x + 4) = 4
y = x + 4
2x + 4 = 4
y = x + 4
2x = 4 - 4
y = x + 4
x = 0
y = 0 + 4
x = 0
y = 4
y = x + 4
x + (x + 4) = 4
y = x + 4
2x + 4 = 4
y = x + 4
2x = 4 - 4
y = x + 4
x = 0
y = 0 + 4
x = 0
y = 4