What is wrong with this

Answer 1

I'm guessing your code is:
if(int=1;i<=15;i+=3)
sum+=i;
cout<<"sum="<<sum;

Your mistake: "if(int=1;" should be "for(int i=1;".
If your code doesn't have "int sum=0;", it must be added prior to your code above.

Answer 2

The code is fragmented please link it or paste it into the description