-3 1/2 -(-12 1/4)
-3 1/2 + 12 1/4
-3 1/2 = -7/2 = -14/4
12 1/4 = 49/4
-14/4 + 49/4 = 35/4
35/4 = 8 3/4
A quick way to convert a mixed number to improper form (improper form makes it alot easier to add, subtract, and more), is like this:
a b/c
((a*c)+b)/c
This will get you the improper form of a fraction.
I hope this makes sense.