You have to make both numbers into improper fractions, so 8 4/5 becomes 44/5 (8*5+4) and 5 1/10 becomes 51/10 (10*5+1). Next, You have to get a common denominator, which will be 10. Multiply the top and bottom of 44/5 by two, and you'll get 88/10. Now you can subtract 88/10-51/10 to get 37/10. This can be reduced to 3 7/10.
Hope this helped!