double num [i]={10.62, 7.23, 15.17, 20.15, 9.62};
double num [j]={8.51, 6.58, 7.35, 15.12, 2.05};
Your instructions told you to define 3 arrays, one of which is current, one of which is resistance, and one of which is voltage. You tried to define two arrays, both named num.
Change the words num in the above arrays to current and resistance, and change i and j to 5.
For instance, double current[5] = {10.62, 7.23, 15.17, 20.15, 9.62};
#include<conio.h> I believe conio.h is deprecated and should not be used at all
as LB has already stated your calcVolts function should not return a value of any kind double calcVolts(double current[5],double resistance[5],double volts[5]);
it should look like this void calcVolts(double current[], double resistance[], double volts[], constint SIZE);
you only need to call the calcVolts function once so the first for loop is not needed