We know that the temperature to start off is 35, so in an equation y=mx+b with x representing the time in hours , m being the slope, and y being the temperature, when x=0 we have y=b, and since the temperature is 35 at the start, y=36=b, making our equation y=mx+35. Next, since we lose 4 degrees every hour, we have the slope, or the change in temperature every hour, equal to -4, making our equation y=-4x+35. We want to find out how long it will take for the temperature to reach -1 degrees, so we plug -1 in for y (temperature) to get -1=-4x+35. Subtracting 35 from both sides using the subtraction property of equality, we get -36=-4x. Next, we divide both sides by -4 to get 9=x=how many hours it will take to reach -1 degrees