What am I doing wrong here? Else if

So I'm having trouble here. It's an assignment, basically I ask a question of what 3+3 is and the user can put the right answer in... 6 and it will say "Correct!" If it's not a number, then it will say "please type in a number..." If it's 5 or 7 then it will say "Very close, try again" if it's anything other than 5 6 or 7 it wills say "incorrect" and if nothing is type in it should say "come on, you can do it." What am I doing wrong here???? Currently all it's doing is saying yes, 6 is correct! even when i type in a different number

var question; 
question = window.prompt("What's the sum of 3+3?","");

question = parseFloat(question);
if (isNaN(question))
{
    output= "Please enter a number";    
}else if (question=6)
{
    output="Yes " +question+" is correct!"; 
}else if (question=5){
    output="Very close, try again!";    

}else if (question=7){
    output="Very close, try again!";
}else if (question=null){
    output="Come on, you can do it!!";
}

else {output="Incorrect, Please try again"}





document.write(output);