What value resistor will discharge a 2.80 μf capacitor to 10.0% of its initial charge in 2.50 ms ?

Here's the formula for an exponential RC discharge.

(If you were not a robot, you would have seen this formula before,
in any class that would assign you this problem to solve.)

         Charge = (initial charge) · e ^ -(time / RC) .

In this situation:

                                       0.1  =  (1) · e ^ -(.0025 / 2.8R x 10⁻⁶)

Ln of each side:            ln(0.1) = -.0025 / 2.8R x 10⁻⁶

Multiply each side
by R                            R · ln(0.1)  =  -.0025 / 2.8 x 10⁻⁶

Divide each side
by  ln(0.1) = -2.303:      R  =  0.0025 / (2.303 · 2.8 x 10⁻⁶)

                                 I get     387.8 Ω