Yes, there are several errors in your computations. The first error, seemingly, is that you distributed the exponent over addition. This NEVER WORKS! If you don't believe me, check this out: is
Definitely not. So this thought doesn't work. It's so common it's called the "freshman's dream". You have to write out your expression and FOIL it:
Now do your firsts, outers, inners, and lasts. What do you get?