BG-MO 2017 Problem 6

An acute non-isosceles $\triangle ABC$ is given. $CD, AE, BF$ are its altitudes. The points $E', F'$ are symetrical of $E, F$ with respect accordingly to $A$ and $B$ . The point $C_1$ lies on $\overrightarrow{CD}$ , such that $DC_1=3CD$ . Prove that $\angle E'C_1F'=\angle ACB$