Determine the capacitance of the capacitor.

I just had the most remarkable experience when I read your question ... a formula popped into my head that I haven't used or even thought about in years !  It's the formula for the charge on a capacitor in terms of the voltage across it:

Q = C · V

Charge = (capacitance) · (voltage)

This is exactly what we need in order to answer this question !

For a graph of charge vs voltage (which we have here !), that formula Q=CV is in slope-intercept form, and it says that the capacitance is the slope of the graph.  How awesomely convenient !

The slope of the whole blue line on this graph is

Slope = (0.025 Coulomb) / (50 volts)

Slope = 0.0005

Capacitance = 0.0005 Farad  or  Capacitance = 500 microfarads

total distance/total time

(85 + 82)/(5+3)

167/8

20 just over