Answer: B. (3x + 6) - x - x
Step-by-step explanation:
To find the width of the door we need to take the upper side of 3x+6 and substract the measurements of the bottom side (the part that is not the door):
From the image, we can see that when we take the red side and substract the blue sides we will get the width of the door:
Thus, the answer is option B.