Differentials in Linear Approximation

Harry, on his way to Hogwarts, arrived at platform $9^{\frac34}$. While waiting for the train, he became curious as to what the value of $9^{\frac34}$ actually was. Sadly, because it's a muggle device, he didn't have a calculator on hand. Luckily, Harry knows how to use differentials. After, a couple minutes, what value did Harry get out?