Two infinitely large sheets of charge are vertical, parallel to each other, and 24.0 cm apart. One of the sheets has a charge density of +4.60 x 10 -7 C/m2. The other sheet's charge density is unknown. Exactly half-way between the two sheets is a small charged mass on the end of an insulating thread. The mass is 2.30 x 10 -7 kg, the charge is -8.20 x 10 -12 C, and the thread is 6.40 cm long. In equilibrium, the thread makes an angle of 9.50°, relative to vertical, away from the unknown sheet. Calculate the unknown sheet's surface charge density.