There is a real issue with how to interpret the information given in the problem. Most specifically, what that meter is showing. If it is showing RMS voltage, then the problem is inconsistently overdefined. If it is showing instantaneous voltage, then you don't know what the instantaneous voltage output from the AC source is because you don't know the time (but you do have enough information to find it. If, as it appears crutschow might be interpreting it, that is not a meter but a second DC source, then the resistor R1 is effectively removed and the voltage across the cap is just the difference between the two sources. Given the low impedance of C1 (about -j16Ω), the impedance of the two sources may be a factor, but unless they are more than an ohm or two combined, the effect will be minimal.Just apply simple KVL, U have 3 voltages , two of them are known and one is unknown and the sum of the voltages would equal zero.
I think It's very easy, U only need to take care of applying KVL and the sign of each voltage!!!