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107 degree + ? = 180 degree (adjacent angles on stright line )

  ? = 180-107 

     = 73 degree 

the angle next to 107 degree is 73 degree 

since the angle next to 107 degree is in the triangle ,

we can use angle sum of triangle to solve this problem

 the angle oppsite to angle A degree is  

 a=a ( vertical oppsite angle )

therefore the angle oppsite to a degree is also a degree

 

 

after that we finall need to find the sum of a + b

 

you should know that the sum of the angle degree in a triangle is 180 degree , right ?

 

threrefore 

 a degree + b degree + 73 degree = 180 degree (angle sum of triangle)

                                                  a+ b= 180-73

                                                        =107 degree

therefore the sum of a degree + b degree is 107 degree

I have a more direct solution than you. 

 

a = a (vertically opposite angle) <-- This means the angle in the triangle is also 'a degrees'

a + b = 107 (exterior angle of triangle)

 

Finished!! :)