Ok, so whenever you have just "n" in an equation, it is really 1n. Lets say "n" is the number of apples on your tree. This is saying you want to give one apple (an "n") to a friend, and you have 4 apples (4n) on the tree, so how many apples do you have left?
-1n+4n would then be simplified to 3n, because -1+4 is 3.
What you can take away from this problem is that whenever you have a variable ("n" or "x" or "y", etc), without a number to multiply it to in the front, it is the same as putting a 1 in front of it. So:
n+2n =3n (same as 1n+2n=3n)
and -3x-x= -4x (same as -3x -1x= -4x).