Answer
1) first coordinate
2) first
3) second
Work Step by Step
Given a set of inputs $X$ (Domain) and a set of possible outputs $Y$ (Range) as a set of ordered pairs $(x,y)$ where $x∈X$ and $y∈Y$, a function is a relation from the Domain to the Range where each input $x$ (first coordinate) is related to exactly one output $y$ (second coordinate) such that there can only be one ordered pair with the same value of $x$ resulted.
