You just have to try them all:
Y=x+3 ≈ 6=3+3 ≈ 6=6 √
Y=x+3 ≈ 8=4+3 ≈ 8=7 X
So the first function doesn't work
Y=2x ≈ 6=2(3) ≈ 6=6 √
Y=2x ≈ 8=2(4) ≈ 8=8 √
Second function works
Y=x+4 ≈ 6=3+4 ≈ 6=7 X
Third function does not work
Y=12-x ≈ 6=12-3 ≈ 6=9 X
Fourth function doesn't work
So your answer is Y=2X :)
