Since you know that a=9 just replace a with 9
your expression is
a-7/8
it becomes
9-7/8
Now we can put 9 over 1 and subtract the fractions
9/1-7/8
you need to find a common denominator in order to subtract the fractions. The common denominator for 1 and 8 is going to be 8
multiply both the top and bottom of the first fraction by 8
73/8-7/8
then subtract the numerators or top numbers
73 minus 7 is 66
66/8 is your answer
you can convert that to a mixed number if you want. 8 goes into 66, 8 times with a remainder of 4
8 4/8
that can be reduced to 8 1/2