All triangles equal 180 degree.
Given, you have 55 degree and 70 degree.
180 degree - 55 degree = 125 degree
125 degree - 70 degree = 55 degree remaining.
Now we know the remaining angle equals 55 degree
55 degree = 9x + 10
55 - 10 = 9x
45 = 9x
x = 45/9
x = 5