In a chemical reaction, the equilibrium constant refers to the value of its reaction quotient at chemical equilibrium, that is, a condition attained by a dynamic chemical system after adequate time has passed, and at which its composition has no measurable capacity to undergo any kind of further modification.
The given reaction is: HCN (aq) + OH⁻ = CN⁻ (aq) + H2O (l)
The equilibrium constant = product of concentration of products / product of concentration of reactants
(Here, H2O is not considered as its concentration is very high)
So, Keq = [CN⁻] / [HCN] [OH⁻]