An owner of a home in the Midwest installed solar panels to reduce heating costs. After installing the solar panels, he measured the amount of natural gas used y y ( in cubic feet) to heat the home and outside temperature x x (in degree-days, where a day's degree-days are the number of degrees its average temperature falls below 65 oF o F ) over a 23-month period. He then computed the least-squares regression line for predicting y from x and found it to be: yˆ= y ^ = 85 + 13x. How much, on average, does gas used increase for each additional degree-day?