Estimate of a complete rational trigonometric su

Supposef is a polynomial of degree $n\ge3$ with integral coefficientsa $a_0,a_1,\dots,a_n$; $q$ is a natural number; ($a_1,\dots,a_n, q)=1$ $f(0)=0$. It is proved that
$$\Bigl|\sum_{x=1}^qe^{2\pi if(x)/q}\Bigr|<e^{5n^2/\ln n}q^{1-1/n}.$$