Claudia, let's recall that this a case of Alternate interior angles and they are equal.
Therefore:
7x - 115 = 3x - 35
Like terms:
7x - 3x = -35 + 115
4x = 80
Dividing by 4 at both sides:
4x/4 = 80/4
x = 20
Let's prove x = 20 is correct:
7 * 20 - 115 = 3 * 20 - 35
140 - 115 = 60 * 35
25 = 25
