1) Variables:
D: distance away from the surface
T: temperature
2) the b-value of the y-intercept for your equation is 15, because it's the surface temperature, that is, at D = 0, T = 15
4) the slope is: -6.5, because temperature decreases approximately 6.5 degrees Celsius for each kilometer traveled away from the surface
5) Equation:
T = -6.5D + 15