View Compilation Error

0_0_29990920_6466.cpp: In function 'long long int mul(long long int, long long int, long long int)':
0_0_29990920_6466.cpp:55:5: error: 'a' was not declared in this scope
  _a=a,_b=b,_m=mod;
     ^
0_0_29990920_6466.cpp:55:10: error: 'b' was not declared in this scope
  _a=a,_b=b,_m=mod;
          ^