A method usually terminates when the statment is returned. You will need to encapsulate the two objects. Java only allows you to return one type.
public int returnNP(int V){
n = -F/E;
int P = (n*R*T)/V;
// ideal gas Law
Return n, P; // will not work
}
int[] nAndP = new int[];
public int [] returnNR(int V){
n = -F/E;
int P = (n*R*T)/V;
// ideal gas Law
nAndP[0] = n;
nAndP[1] = P;
Return nAndP; // will work
}