Answer:
 
- When Three sides of the triangle are known 
- Two sides of the triangle is known and corresponding angle is included 
- Two angles known and their included side given
- Two angles known and the length of a non included side is given
Step-by-step explanation:
The measure that will determine a  unique triangle are 
- when Three sides of the triangle are known
- Two sides of the triangle is known and their corresponding angle is included 
- Two angles known and their included side given
- Two angles known and the length of a non included side is given
unique triangles are termed unique because they can be drawn only using one way
 
        
             
        
        
        
Note the equal sign. What you do to one side, you do to the other. Isolate the a.
Add 4 to both sides
 a- 4 (+4) = 15 (+4)
a = 15 + 4
a = 19
19 is your answer for a
hope this helps