Answer is: formula of the complex is [Cr(NH₃)₂(SCN)₄]⁻.
This complex has negative charge (-1) because chromium (central atom or metal) has oxidation number +3, first ligand ammonia has neutral charge and second ligand thiocyanate has negative oxidation number -1:
+3 + 2·0 + 4·(-1) = -1.
