The solution at $x=1$, $t=1$ of the partial differential equation $\frac{\partial ^2 u}{\partial x^2} = 25 \frac{\partial ^2 u}{\partial t^2}$ subject to initial conditions of $u(0) = 3x$ and $\frac{\partial u}{\partial t}(0) =3$ is _______

1. $1$
2. $2$
3. $4$
4. $6$

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