From Maxwell's equations, we know that $\nabla \cdot \mathbf{E}=0$, so the first term vanishes. We are left with $\nabla \times(\nabla \times \mathbf{E})=-\nabla^{2}\mathbf{E}$. Now, we can use the equation $\operatorname{curl} \mathbf{E}=-\frac{1}{c}
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