Respuesta :

Answer:

90

Step-by-step explanation:

a triangle's angles all add up to 180. take 51 and 39 away from 180 and you get 90

Answer: 90 degrees

Step-by-step explanation:

Theorem:

a+b+c=180 degrees

39 degrees + 51 degrees + x degrees = 180 degrees

39 + 51 = 90

180 - 90 = 90