View Compilation Error

0_0_9750962_28052.cpp: In function `long long unsigned int mul(long long unsigned int, long long unsigned int, long long unsigned int)':
0_0_9750962_28052.cpp:23: error: `c' undeclared (first use this function)
0_0_9750962_28052.cpp:23: error: (Each undeclared identifier is reported only once for each function it appears in.)