My understanding of the math I learned on Khan

1) I watched the Beauty of Algebra video.
I learned that math is the universal language. I already had an understanding that math can be the voice for people that speak another language from me but I had no idea that it could possible be the language that may connect us to the rest of the universe. That is so exciting! The video taught me that in algebra x or any symbol can mean lots of things and if you divide x by y which can also stand for anything I want, lots of great things can happen. I learned that letters can stand in for any item or symbol to make an equation like y=p(x) also in physics f=m(a) which is force equals mass times acceleration. It's the same equation only using different symbols. The beauty of this equation is that it can be used in any application. And it's not so hard to understand.
Algebra is not very hard for me but its getting a better understanding of the topic and remembering what I've learned is the hard part. I'm excited to learn more.

2) Understanding variables video:

As I watch the videos on variables I didn't know, or maybe I forgot I knew, that a negative number times a positive number equals a negative number. The practices stumped me twice. But it didn't stop me! I tried two more problems. Here's one I did:
c=4 and d=8
-10(c) +3(d) +5
I got -49 but the answer is -11 and I don't understand how they got that answer

Another problem I did looked like this:
b equals 6(6) and a equals 1
6b squared + 5a-5
The answer is 216
This problem looked like it was going to stump me but guess what? I REMEMBERED! I remembered that squared is multiplication.

3) Combining like items:
While watching the video I learned that 2x+3x=5x and then I learned that I can not add different items together such as y's to x's. Such as 7y+2x+3x+2y= 9y+5x. That was cool.
Now I did a problem with combining items and I don't understand how they got their answer. My answer was -1b+3. The computer said the simplified answer was just b+3. Why?