Here a pivotal quantity is where is the true slope of the regression equation (unknown), is its least square estimate and is its estimated standard error. And T has t-distribution with n-2=15-2=13 degrees of freedom (n is the sample size). We have that , and because we want the 95% confidence interval, we should use the 2.5th quantile of the t distribution with 13 df, this value is -2.16 and the 95% confidence intervale is given by , i.e., . Therefore, the 95% confidence interval explicitly is (-5.666, -0.926)
Okay, I can exactly show my work. but you can answer this by counting the muber of spaces between points A and C. So the answer should be A 13, my bud.