Hello there!
Let's take a look at our geometrical figure. It's a triangle; therefore, the angles of the triangle - regardless of how big or small - must be equivalent to 180 degrees.
Since we have 2/3 given angles, let's sum up our current angles and subtract that sum from 180.
50 + 75 = 125
180 - 125 = 55
This means our missing angle value, x, is 55 degrees.
I hope this helps!