Question

For which function will both ordered pairs (3,6) and (4,8) be solutions?

Answer 1

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 :)