That is a question about angles.
The first thing that we should to note is that a line do a straight angle. A straight angle is an 180° angle.
So, we can say that the sum of the angle a and the angle of 157° is equal to a straight angle. Let's make and solve a equation:

Therefore, the missing angle is 23°.
I hope I've helped. :D
Enjoy your studies. \o/