你在main函数中的这一行语句void equation(double a,double b,double c);是函数的声明不是调用改为equation(a,b,c);就行了
main函数中void equation(double a,double b,double c);改为
equation(a,b,c); // 函数调用
void equation(double a,double b,double c);是声明
将void equation(double a,double b,double c);改为void equation(int a,int b,int c);