how can i solve this???

HI Angel Milko Santiago,

To solve this, you'll need to convert the words into mathematical symbols.

In your case, decreased by means subtraction. So we can start by replacing that with the minus sign:
A number - the difference between the cube of the number and 10.

Now, let's replace a number with a variable, say n:
n - the difference between the cube of n and 10.

The difference between also means subtraction, so let's apply this:
n - (the cube of n and 10)

And refers to addition:

n - (the cube of n + 10)

And the cube of shows to the third power (I will use ^x to show to the x power):
n - (n^3 + 10)

That's it, you've expressed it as a variable expression.

The equation can't be simplified at this point. 

@Admiral Betasin, then how should I answer that question? I dont understand

If you're asking regarding the equation n - (n^3 + 10), there's no way of finding a specific value of n unless it's turned into an equation.

This also cannot be simplified. You can distribute the negative, but it won't get your much farther. Only to n - n^3 - 10

What about this one?
Translate into a variable expression. Then simplify the expression.
2. 8 more than the sum of a number and 11
How can i solve it?

8 more means plus, so we can rewrite this:
The sum of a number and 11 + 8

Now, a number is the variable in the expression, so we can replace it with a variable, which I will call n:
The sum of n and 11 + 8

The sum is addition, so:
n + 11 + 8

Now, 11 + 8 is 19, so simplify that, and you're done:
n + 19

Thankyou. @Admiral Betasin, can i ask more questions?

Since I've showed you the concept, I'd like to see if you can solve the problem on your own now.

If you're stuck, you can ask me for help.

How can i learn about this in khan academy?

I was unable to find any content matching, but you might be able to find something if you use the search bar.