We can think of the constant term in a function in terms of x, for example in x^2 + 3x + 2 as 2 being multiplied by x^0. Anything to the power of 0 is equal to one, so in our example we would have 2 * x^ 0 which is the same as 2 * 1 which is 2, but this trick allows every term to have x of a certain power. Differentiating first multiplies the power of the x term with the coefficient, then takes one away from the power- with the constant term, multiplying the coefficient, the 2, by 0, will cause the whole term to disappear before we get to the second step.