From the problem :

Adding and substracting fractions need to have the same denominator.
Express 1 as a fraction with 8 as the denominator.
Note that :

1 is equal to 8 divided by 8
Replace it from the equation :

Since the denominators are now the same, perform the operation for the numerator, in this case 8 - 7 = 1
then copy the denominator.
The result will be :

The answer is 1/8