we know that alternate angles are equal.
Therefore the two angles labelled x are equal.
Also, the two angles labelled y are equal.
We know that x, y and z together add up to 180 degrees, because these together is just the angle around the straight line.
So the three angles in the triangle must add up to 180 degrees.

If in the above diagram x = 79 ° and y = 44 °
Since the total of all the 3 angles is 180 °
Using properties of traingles
x = 79 °
y = 44 °
Since x + y + z = 180°
Thus 79 ° + 44 ° + z = 180°
Thus z = 180° - 79 ° - 44 °
Thus z = 57 °