The measures of the angles of triangle abc are given by the expression and the table

In a triangle all the angles add up to 180  degrees.So we know that
(6x-1)+20+(x+14)= 180 
6x+x+14-1+20=180
    v        v
  7x +   13 + 20= 180
                 v
        7x + 33=180
               -33   -33
                  0     147
        7x/7    =   147/7
           x      =       21
x=21
Then you substitute:
(6x-1)       (x+14)
(6×21-1)    (21+14)
    v                  v
  126-1           35
125 degrees / 35 degrees
125+35+20=180

The measurement of angle A=125 degrees
the measurement of angle C= 35 degrees