Well, you can try this:

int findSum (vector < int > v){
int sum = 0;
if(v.size() <= 0) return 0;
else{
position = v.size() - 1;
sum += v.at(position);
...

put
total += length,
total += length/3.208,
total += ((length / 12) * 3.2808);

in their respective places. Note D += A is shorthand for D = D + A
Dont forget to terminate the statements.