I have no idea what my teacher is talking about or asking for can anyone help thanks!

f(x) =-0.12x - 1.4 ?

AND

this word problem is driving me insane!

When going more than 38 mph the gas mileage of a certain car fits the model y=43.81 - .395x where x is the speed of the car in mph and y is the miles per gallon. based on this model at what speed will the car average 15 miles per gallon round to the nearest whole #