If you work it out:

2(-1)-1(4)

(-1)^2

is equal to -6. However if we cancel the (-1) on top so it's just 2(1) and cancel (-1)^2 to become just (-1) we get:

2(1)-1(4)

(-1)

which is 2. The problem is you have that " -1(4) " bit at the end. If it were just:

2(-1)

(-1)^2

Then you could definitely cancel the (-1). However, the " -1(4) " part screws it up! Hope this helps.