What’s SI units and SigFigs???

Significant figures have a few rules about 0s and you should research but simply, for example say round 27.879 to 3 significant figures you round up until you have 3 numbers that aren't 0s so it would be 27.9.

SI units are the international system of units.

SI units refer to the International System of Units. These are the standard units that measurements are typically given in. For example, the SI unit for time is seconds, meters for distance, kg for mass, etc.

Significant figures are the number of notable digits given for a measurement. For example, let's say you measured a piece of metal to be exactly 3.00 meters long. The value 3.00 has three significant figures, where as 3 only has one significant figure. Larger amounts of significant figures indicate a more precise measurement. In this case, by writing 3.00 instead of 3, you're telling me that you measured that piece of metal to be precisely 3.00 meters long.

Electric potential energy is defined as Ep=Q*V where Q is the magnitude of the charge and V is the potential difference. So when a charge moves between the points that have a potential difference, it's energy changes. 

In our case: 

Q=2e=2*(-1.6*10^-19) C
V=75 V

Ep=(-3.2*10^-19)*75

Ep=-2.4*10^-17 J

The change in potential energy of the charge is -2.4*10^-17 J