everytime i try to add the for loop, the task one stops working

You're reasonably close, but things are sort of misplaced and you have two closed curly braces but only one open curly brace. And you have two open parentheses but only one closed parentheses. Note that when it says a previous task has stopped working, it generally means that the current code you're trying has introduced a syntax error. Here's what it's looking for:

int mathTotal;
bool isComplete;

for(int i = 5; i <= 25; i++) {
  mathTotal += i;
}

isComplete = true;

We start the loop with i equal to 5 and then we go all the way up to 25. Every time through the loop we increment i by one. We take mathTotal and add what it is currently plus i. So the first time through the loop it'll be 5 because mathTtoal isn't initialized to anything. The next time through we'll add 6 so the result will be 11. The next time through we'll add 7 to the previous value (11) and get 18. This happens all the way up until i is greater than 25. At that point the loop will exit. When it does, we set the value isComplete to true. Because... it's complete! :)

Hope this helps!