345 rounded to the nearest 10 is 350.

First, you look to the tenth place, which is 4. 
Then you look to the number to the right of that, which is 5.

Remember this rule:
4 and lower goes down,
5 and higher goes up.

Since it's 5, we go up to 0.

And that is how you get 350.
The answer is 350.

In rounding off numbers, look at the digit at the right of the number to be rounded.

If it is 1,2,3,4, retain the digit and replace other digits that follow with zeros.

If it is 5,6,7,8, or 9, add 1 to the digit to be rounded and with zeros after it.