Let the cost of one sheep and one cow be x and y respectively. Therefore,
5x+5y=129 10x+17y =177
Solving the two equations, Multiply the first equation by 2 and subtract the second from the results; 10x+10y = 258 10x+17y = 177 0x-7y = 81 => y= $-11.57 (cannot be a negative value). The details of the question need to be checked.