Respuesta :

BrainyAlice28

Answer:

D

Step-by-step explanation:

You can see, MN consists of the three lines in the picture. So you just need to sum them tougher in an equation with it's original value 81.

5 + 3x - 8 + 4x = 81

7x = 81 + 3

7x = 84 / : 7

x = 12