#include<iostream>
using namespace std;
typedef long long inta;
int extend_gcd(inta a,inta b,inta &x,inta &y,inta &gcd)
{
if(b==0)
{
x=1;
y=0;
gcd=a;
}
else
{
extend_gcd(b,a%b,x,y,gcd);
int temp=x;
x=y;
y=temp-a/b*y;
}
}
int gcd(int a,int b)
{
if(b==0)
return a;
else return gcd(b,a%b);
}
int main()
{
inta n,d,x,y;
int times;
cin>>times;
while(cin>>n>>d>>x>>y)
{
inta a=d;
inta b=n;
inta c=y-x;
inta s=gcd(a,b);
if(c%s!=0)
{
cout<<"Impossible"<<endl;
}
else
{
inta x0,y0,q;
a/=s;
b/=s;
c/=s;
extend_gcd(a,b,x0,y0,q);
x0*=c;
if(b<0) b=-b;
x0=(x0%b+b)%b;
cout<<x0<<endl;
}
}
}
// 和青蛙的约会一样
