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Yes, the reason we must add here is that the rule for finding a p-value for a left-tailed test is to find the area to the left of the test statistic. Since the test statistic ended up being a positive value, the test stat is on the right side of the curve. To find the area to the left of the test stat here, we must find the area from that value 1.58 all the way over to the left side's negative infinity. The whole left half of the cure is 0.5000 and the part from 1.58 until the middle of the curve is found from the table. The two parts added together make up the p-value. If the test statistic had been on the left-hand-side of the curve, you would have been looking for a tail area, and it would have been found the way you expected.