They used more cleaner in 2012 because the green line is higher in the year 2012 than it is in 2015.
The equation of the line is
.
Solution:
Given points are (2, 7) and (0, 4).
Slope of the line formula:

Here 
Substitute these in the given formula.



Point-slope formula:

You can take any point in the given line. Here the point is (2, 7).

Multiply by 2 on both sides of the equation.


Add 14 on both sides of the equation.

Divide by 2 on both sides, we get

The equation of the line is
.
Yes it’s a function since every input has at least 1 output and there’s not multiple lines coming from the input side