Finding net electric potential, i got it set up right, but somthing is f'ed up

[mr_coffee]

Hello everyone I think i set this up properly but the answer is still wrong. I tried 2 different ways, both wrong.
In Figure 24-33, what is the net electric potential at point P due to the four particles, if V = 0 at infinity, q = 5.50 fC, and d = 3.00 cm? Figure 24-33
I used potential due to a group of point charges. V = The sum of all the point charges.
V = 1/(4pi*Eo)(q1/r + q2/r + q3/r + q4/r);
V = 1/(4pi*Eo)(5.50/.03 + 5.50/.03 + 5.50/.03 + 5.50/.03);
V = 6.59x10^12 V;
Which was wrong so i noticed that 2 of the charges q, where negative, so I tried:
V = 1/(4pi*Eo)(5.50/.03 + 5.50/.03 + (-5.50/.03) + (-5.50/.03));
V = 0 V; Wrong again.
What am I doing wrong? Is it because q = 5.50 fC? I have no idea what f stands for? is it 10^-15 or what?

 

[member 553083]

The f in 5.50 fC stands for femto, which means 10^-15. Therefore, q = 5.50 x 10^-15 C. With this correction, your equation should be:V = 1/(4pi*Eo)(5.50 x 10^-15/.03 + 5.50 x 10^-15/.03 + (-5.50 x 10^-15/.03) + (-5.50 x 10^-15/.03));V = 0 V