I'm 27 years old and use to weight 143 and now i'm 115. i'm trying to go back to 143 but i'm having trouble gaining weight what can i do?

Calories & protein: The addition of calorie dense foods such as nuts, whole grain pastas, raisins, whole milk, pure fruit juices, butter, avocado, and complex carbohydrates is a healthy way to add more calories to your diet. Lean proteins are also important. Good luck

Several things: First you want to make sure you've had a reasonable medical evaluation to rule out concerning reasons for weight loss/inability to gain weight. If that's not remarkable, it means that you will have to take a two pronged effort at weight gain. It should focus on eating high quality calorically dense food(including health fats, not junk food) and remaining active but not excessively.