In a study of copper bars, the relationship between shear stress in ksi (x) and shear strain in % (y) was summarized by the least-squares line y = −20.00 + 2.56x. There were a total of n = 17 observations, and the coefficient of determination was r² = 0.9111. If the total sum of squares was ∑(y_i - y̅)² = 234.19, compute the estimated error variance s².