The equation of this line in slope intercept form is y= 3/7x

bear in mind that, though the square has two valid roots, one negative and one positive, the hypotenuse is just a radius distance, and therefore is never negative.
Answer: The third graph.
The difference between temperatures for every increase (1km) in altitude is consistent (6.5), meaning that it is a straight line.
Then, you would have to match the x and y values on the charts to the coordinates on the graph. Ex: (8.5, 1), (2, 2), (-4.5, 3) etc.