Okay, let me explain.
Basically, you need to complete the square.
So you have,
+14x+__________ to equal a perfect square.
Your middle term is 14. So, what you need to do is divide fourteen by 2 which equals 7. What does 7*7 or 7 squared equal? It equals 49.
The missing constant term is 49.
I hope this helped; feel free to ask any follow-up questions.