Two Parallel Square Metal Plates $8.4 \mathrm {~cm}$ On Each Side, Are

Two parallel square metal plates, $8.4 \mathrm {~cm}$ on each side, are $2.5 \mathrm {~mm}$ apart and carry equal but opposite charge uniformly distributed over their facing surfaces. How much excess charge is there on each plate if the electric field between the plates has a magnitude of $2.0$ $\times 10 ^ { 6 } \mathrm {~N} / \mathrm { C } ? \left( k = 1 / 4 \pi \varepsilon n = 8.99 \times 10 ^ { 9 } \mathrm {~N} \cdot \mathrm { m } ^ { 2 } / \mathrm { C } ^ { 2 } \right)$