The matrix $$A = \left[\begin{array}{ccc} 3 &0 &-4\cr -4 &-1 &4\cr 2 &0 &-3 \end{array}\right]$$ has two real eigenvalues, $\lambda_1 = -1$ of multiplicity $2$, and $\lambda_2 = 1$ of multiplicity $1$. Find an orthonormal basis for the eigenspace corresponding to $\lambda_1$.