You used the += in your original code above. Is there something confusing you about how you used it above other than the fact that you are referencing the non-existent ourStr variable?
The += operator is just a shortcut for variable = variable + sum. Here’s an example:
var x = 1;
var y = 2;
x += y; //this will output 3
//it is the same thing as
x = x + y;//this will also output3
//You can also use it to add strings as what the challenge is basically
//asking you to do