Let's begin with recalling that a linear pair is a pair of adjacent angles with non-common opposite sides.
Remember that the measures of these two angles always have a sum of 180^(∘) as together they create a straight angle. Now, let's look at the given picture.
As we can see, ∠ 3 and 43^(∘) form a linear pair. Therefore, we can write that the sum of the measures of these angles is equal to 180^(∘).
