Linear Pair
If two angles form a linear pair, it means that they add up to 180 degrees. Because their sum is 180, when setting up an equation to solve for x, we set the right side equal to 180.
On the left side, we have the angles that form the linear pair, i.e., x and 50.
Overall, I end up with the following equation:

Now to solve for x, subtract 50 on each side:

Hence, x = 130.

