I have worked this a few times and keep ending up with 45/7 for the answer. The book says -9 is the answer. Any help is appreciated!

When a certain number is added to the numerator and subtracted from the denominator of the fraction 3/8, the result is 6. What is the number?

I have tried the following :

3+x/8-x = 6

(8-x) 3+x/8-x = 6 (8-X)

3+X = 48-6x

45=7x

X=45/7