Hi there!
Since we know that the sum of all 3 measures of a triangle is always equal to 180 degrees, and that supplementary angles are equal to 180 degrees when added, we can solve by first finding the supplement to ?.
75 + 40 + ? = 180
180 - 75 - 40 = ?
105 - 40 = ?
? = 75
Now, we find the actual? .
Since 75 + ? = 180,
180 - 75 = 115
? = 115
So, your answer is 115°.
Hope this helps!