Suppose
\[
u(x, t) = \frac{1}{2} \left[ g(x + ct) + g(x - ct) \right]
\]
is a solution of the following initial value problem of the wave equation
\[
u_{tt} = 9 u_{xx}, u(x, 0) = g(x), u_t(x, 0) = 0.
\]
Then the value of \( c^2 \) is \(\underline{\hspace{2cm}}\) (round off to 2 decimal places).