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
