postfix to infix & the result. stack C++

Hello! I have a homework in which i should do this:
Input
x+-3129
Output:
3x1+2-9=-4
I wrote this code and my output is : 3-1+2x9
Can somebody help me to find the problem and a way to calculate the expression in code?

#include <iostream>
#include<stack>
using namespace std;

bool isOperand(char c)
{
    if( (c >='0' && c<='9') )
        return true;
    else
        return false;
}

string  PrefixtoInfix(string prefix)
{
    stack<string> s;
    for(int i = prefix.length()-1;i>=0;i--)
    {
        if(isOperand(prefix[i]))
        {
            string op(1,prefix[i]);
            s.push(op);
        }
        else
        {
            string op1=s.top();
            s.pop();
            string op2=s.top();
            s.pop();
            s.push(op1+prefix[i]+op2 );

        }
    }
    return s.top();
}

int main() {

    string prefix,infix;
    cin>>prefix;
    infix=PrefixtoInfix(prefix);
    cout<<infix;

    return 0;
}