What is x + y = 3
becomes y = 3 - x
so if you put a value of 1 in place of x you get y = 3 - 1 therefore y = -2 when x is 1
when x = 2 y = 3 - 2 therefore when x is 2, y is -1
x = 3 y = 3- 3 = 0
x = 4 y = 3 - 4 = 1
co-ordinates are
x 1 2 3 4
y -2 -1 0 1
plot these on a graph