The given angles are known as alternate interior angles.
For line L and M to be parallel, the two given angles must be the same.
Set them to equal each other and solve for X.
7x - 7 = 4x +14
Add 7 to each side:
7x = 4x +21
Subtract 4x from each side"
3x = 21
Divide both sides by 3:
X = 21 / 3
X = 7