We know that the triangle is made up of 180°
To find "x" we need to know the two angles of the triangle.
We also know that the supplementary angles are made of 180°
In this case the supplementary angle would the angle were 100° and the missing value is.
Since supplementary angles are made of 180°
180°-100° = 80°
Now we know the two angles of the triangle.
Since the triangle is made up of 180°
180° = x + 70° + 80°
x + 150° = 180°
x = 180° - 150°
x = 30°