Let the third angle of the triangle be y.
y + 64° + 45° = 180° (angle sum of triangle is 180°)
x + y = 180° (angle of a straight line is 180°)
Notice that they both equal 180°. This means that they are the same thing, hence:
y + 64° + 45° = x + y
Since y = y, it is not necessary to write them down. Remove y from both sides of the equation:
64° + 45° = x
Surprise! exterior angle = sum of interior opposite angles.
I hope this clears up any confusion you might have. If not, feel free to leave a comment with your question.
