u see that 80 degree angle in the lower right side...it is congruent to the angle vertical to it....because vertical angles are congruent. So the lower right angle is 80 degrees.
Now , on the lower left side u have ur 123 degree angle. The angle adjacent to the 123 degree angle (the angle we r looking for) forms a straight line and when added, will equal 180 degrees
180 - 123 = 57. So the lower left angle in ur triangle is 57 degrees
ok...the interior angles of a triangle all add up to 180 degrees....
57 + 80 + x = 180
137 + x = 180
x = 180 - 137
x = 43......<==