Answer:
The charge on the negative plate is -Q.
Step-by-step explanation:
If a parallel plate capacitor has the positive plate charged to +Q, the charge on the negative plate will be -Q. The total charge on a capacitor is conserved, so if one plate has a positive charge of +Q, the other plate must have an equal and opposite charge of -Q to maintain charge neutrality. Therefore, the charge on the negative plate is -Q.