Parallel RC with current source
The mean value of the voltage is the same thing as its DC value. The flip side of this is that on average no current goes through a capacitor. This means all of the DC current must pass through the resistor. Calculate the DC current from that waveform, and you should find it simple to calculate the mean voltage from there.

This circuit is simple enough that you can actually solve the differential equation in the time domain to double check this logic. Just split the period into two segments: one where the current source is off, and one where it is linearly decreasing from its max value. If you know the duration of each of these segments, you can calculate the ending voltage as a function of the starting voltage. Then simply set the start value of each to the end value of the other, and solve. Then go back and calculate the average voltage. If it's been awhile since you've solved differential equations you may need to review the basics of solving first order ODEs using integrating factors, but it's not too difficult.