Because a horizontal line is 180 deg.
Use 180 - 70 to get have of DTO Angle deg.
DTO = 110 deg.
70 is across 3x -5
The 3x - 5 is opposite of 70 so the angle is also 70 deg.
So 3x - 5 = 70
Isolate x
Add 5 to both sides of the equation
Should look like this now
3x = 75
Divide by 3 on both sides
Now x = 25