1) From the picture, we see that we have two vertical angles:

2) Now, because of the "Vertical Angles Postulate" from geometry, we know that angles A and B are equal, so equalling the angles and replacing by their expression from above, we get the following equation in terms of x:

3) We must solve the last equation for x, doing that we find:

Steps to solve the equation
0) We have the equation:

1) We want all the terms with x on one side, and the other on the other one. So we pass the term +3x to the right as -3x:

2) Now we pass the term -10 to the left as +10:

3) We sum the terms on each side:

4) We pass the 3 that multiplies the x on the right, dividing the 60 on the left:

5) Finally we make the division and we get:

Answer
x = 20