You can put this solution on YOUR website! i will not say i am 100% sure but i am around 90% sure, i believe that the answer is 2/7.
-If you plug in -7/2 for x you will get f(-7/2)=-1/(-7/2)
-If you have a fraction in the denominator you multiply the fraction by its reciprocal. So it would be (-1/1)(2/-7), you will get (-2/-7) also equal to 2/7