The ammonium cyanide dissociates into ammonium ion and cyanide ion. We have the K_a for the ammonium ion, NH4^+, but need the K_b for the base, CN^-. Recall that K_b = K_w/K_a = 1.61 x 10^-5 for CN^-.
We see that 1.61 x 10^-5 < 5.7 x 10^-5 and the solution will be acidic.