Chapter 15.7, Problem 1RC

Consider the following two acids and their ionization constants:

$\begin{array}{ll}\begin{array}{r}\hfill \text{HCOOH}\\ \hfill \text{HCN}\end{array}\hfill & \begin{array}{l}{K}_{\text{a}}=1.7\times {10}^{-4}\hfill \\ K_{\text{a}}=4.9\times {10}^{-10}\hfill \end{array}\hfill \end{array}$

Which conjugate base (HCOO⁻ or CN⁻)is stronger?