To determine the error made by Brian, we need to analyze his application of the distance formula. The distance formula is given by d = √((x2 - x1)² + (y2 - y1)²). In this formula, the differences in the x-coordinates (x2 - x1) and y-coordinates (y2 - y1) are squared and then summed before taking the square root. Brian correctly subtracted the coordinates to find the differences, and he squared each difference term correctly. Therefore, the error lies in the interpretation of the question. The correct answer is that he did not make an error in the subtraction; rather, he correctly applied the distance formula. However, the answer choice D suggests that he made an error by subtracting instead of adding, which is misleading. The correct conclusion is that Brian did not make any errors in his calculations.