Answer: 
 
y+z=x 
 
Step-by-step explanation: 
 
we know that 
 
The Exterior Angle Property of a Triangle states that the measure of exterior angle of a triangle is equals to the sum of measures of its interior opposite angles 
 
In this problem 
 
Angle y and angle z are interior angles of triangle ABC and angle x is a exterior angle of triangle ABC 
 
therefore 
 
Applying the Exterior Angle Property of a Triangle 
 
y+z=x
Hope This Helps!