The results that follow were obtained from an analysis of data obtained in a study to assess the relationship between percent increase in yield (Y) and base saturation (x1, pounds/acre), phosphate saturation (x2, BEC%), and soil pH (x3). Fifteen responses were analyzed in the study. The least-squares equation and other useful information follow. y^=38.83-0.0092x1-0.92x2-11.56x3, Syy=10965.46, SSE=1107.01

a. Is there sufficient evidence that, with all independent variables in the model, β2 < 0? Test at the α = .05 level of significance.
b. Give a 95% confidence interval for the mean percent increase in yield if x1 = 914, x2 = 65 and x3 = 6.