You are looking for the x values of this "rule". It gives you the y-values so all you have to do is plug the y-values into the rule and solve for x
y = 4 so...
4 = x + 3
4 - 3 = x + (3-3)
1 = x + 0
x = 1
When y is 4 then x is 1
y = 8 so...
8 = x + 3
8 - 3 = x + (3 - 3)
x = 5
When y is 8 then x is 5
y = 5 so...
5 = x + 3
5 - 3 = x + (3 - 3)
2 = x + 0
x = 2
When y is 5 then x is 2
Hope this helped!
~Just a girl in love with Shawn Mendes