Greetings!
Because you are adding 6 and 8 first, 6 + 8 will be in the brackets because they are what is getting multiplied. Then simply add a 3 x in front of the brackets to show the product in the brackets is getting multiplied by 3:
3 x (6 + 8) , or (6+8) x 3
So your answer would be C!
Hope this helps!