Hey there!! 
Let's take the number as 'x'. 
First, we multiply the number by 2 = 2x 
Then, subtract 6 = 2x - 6 
Then, divide by 8 = ( 2x - 6 ) / 4
The resulting value is 8. 
... (2x-6)/4 = 8 
Now, let's multiply both the sides by 8. 
... 2x-6 = 32 
Adding 6 on both sides. 
... 2x = 32+6
... 2x = 38 
... x = 38 / 2 
... x = 19 
Hence, the number we are thinking of is 19. 
Hope my answer helps!