C++, 247 bytes

#include<string> #define G string #define R return using std::G;int I(G s){for(int i= 0;i<s.size();++i)if(s[i]!=s[s.size()-i-1])R 0;R 1;}G S(G s,int i){R s.substr(0,i)+s.substr(i+1);}int P(G s){for(int i=0;i<s.size();++i){if(I(S(s,i)))R i+1;}R-1;} 

Test code example :

std::cout << P("RACERCAR") << '\n';