Suppose the variance in trunk diameter of the giant sequoia tree species is 15.7m^2, while the variance in trunk diameter of the California redwood tree species is 10.6m^2. Let x1 represent the average trunk diameter of four randomly sampled giant sequoia trees, and let x2 represent the average trunk diameter of three randomly sampled California redwood trees. If the random sampling is done with replacement, what is the standard deviation σ(x1−x2) of the sampling distribution of the difference in sample means x1−x2 ?