Great, correct answers have been given, but I want to make sure you understand why those answers are correct and the others are wrong.
For -7,
Not a natural number. Why? Because natural numbers are only positive integers (and sometimes zero is considered).
Not a...

For the train problem, it is a systems of equations problem.
Assume one train, let's call it Train A starts at (0,0), which is the origin. The other train, Train A, starts at
(0,300), where the y-value is the distance in kilometers.
Now, the trains are moving...

Here is another way to solve this problem.
To solve it, what you want to do is isolate one of the x's or y's, plug what you get into one equation, solve for one variable, and then other (phew... that was a lot to write... let me just show you).
Our equations are: 4x + 5y = 12...

I'm a little confused about what you're asking, but maybe this will help.
If you need to create an equation, you can just put your answer as 7, and try some different combinations.
For example: 3 + 4 = 7
Or: 3 + 4 - 0 = 7
Or: (3 + 4) / 1 =7 <-- In this one, you are...

This question may seem confusing because there are parentheses involved. Because you are not multiplying, dividing, or subtracting the two sets of parentheses, just get rid of them!
By eliminating parentheses, we get: -9x2 + 6x + 2 + 4x2 - x - 1.
Now, group like...

If the phone company charges a service fee, then we know that no matter how many minutes the caller talks, they will always have to pay it. Because of this, we know our equation must include +0.70.
This is the tricky part, and you can write this equation two ways. I will show you one of...

First, if you struggle with staying on task, you should always do your most difficult homework first, while you are still focused.
You should always sit at a desk-- this will also keep your mind from wandering. One of the worst things you can do is be on the couch watching TV if you struggle...

If you have two numbers added together, but can be subtracted to be zero, then the two numbers must be the same.
Here's an example:
1+1 =2, but 1-1 = 0!
So, to answer your question,
8+8 = 16, and 8-8 = 0!