C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	string s,w;
	cin>>s;	
	//r1存放带字母的参数的计算结果(未知数系数)
	//r2存放数字的结算结果 (常数系数)
	int i,r1=0,r2=0;
	int f = 1;//遇到=取反 
	char c;//存放字母 
	for(i = 0;i < s.size();i++){
		if(s[i]=='=') f = -1;
		if(isalpha(s[i])&&(i==0||!isalpha(s[i]))){
			if(i==0) r1++;
			else if(s[i-1]=='+'||s[i-1]=='=') r1++;
			else if(s[i-1]=='-') r1--;
		}
		if(s[i]=='-'||isdigit(s[i])){
			w = w + s[i];
			//如果是最后一个，那么w一定是数字 
			if(i==s.size()-1){
				r2 = r2 + f * atoi(w.c_str()); 
//				cout<<w<<" "<<r2<<endl;
				w = "";
			//带字母的参数	
			}else if(isalpha(s[i+1])){
				r1 = r1 + f * atoi(w.c_str());
				c = s[i+1]; 
//				cout<<w<<" "<<r1<<endl;
				w = "";
			}else if(s[i+1]=='+'||s[i+1]=='='||s[i+1]=='-'){
				//数字
				r2 = r2 + f * atoi(w.c_str()); 
//				cout<<w<<" "<<r2<<endl;
				w = "";
			}
		}
	}
	
	//cout<<r1<<" "<<r2<<endl;
	if(r2 == 0) cout<<fixed<<setprecision(3)<<c<<"="<<0.0<<endl;
	else cout<<fixed<<setprecision(3)<<c<<"="<<r2 * -1.0 / r1<<endl;
}