If the value of Ka for HF is 3.5 x 10⁻4, calculate the equilibrium constant for F⁻(aq) + H₂O(l) <--> HF(aq) + OH⁻(aq)

a) 3.5 x 10⁻⁴
b) 1/(3.5 x 10⁻⁴)
c) 3.5 x 10⁴
d) 1/(3.5 x 10⁴)