Lets have a look at the triangle image above.
What relationships do we know exist?
We know that $y+z=180$y+z=180, because Adjacent angles on a straight line are supplementary (they add up to $180$180 degrees).
We also know that $w+x+y=180$w+x+y=180 because The angle sum of a triangle is $180$180 degrees.
Since both of these equations sum to $180$180, we know that they are both equal. This means that we have
Subtracting $y$y from both sides of the equation gives us the result
Have a look at where these angles are positioned on the triangle.
This special relationship is used frequently in solving geometrical problems. We say formally that the: exterior angle of a triangle is equal to the sum of the two opposite interior angles. Can you see how this statement is a descr