Because .
Be careful about using "x" there since it will typically be interpreted as a variable and you mean it as a constant here. It is true that the derivative of e^(ar), with respect to r, where a is a constant, is ae^(ar) so that the derivative of e^(ar)/a, with respect to r, is ae^(ar)/a= e^(ar) so that the antiderivative of e^(ar) (again with constant a) is e^(ar)/a.