(3+5)2

In order to find the answer to this problem, you need to understand what we call the order of operations.  When you are presented with different operations, you must know what order in which to do them.  If people solved math problems by doing different operations in different orders, they would all get different answers, not a good result. 

The order of operations is as follows:

Parentheses

Exponents

Multiplication

Division

Addition

Subtraction

This means that when presented with these choices, you do the operations in exactly that order.  We have a sentence that helps us to remember the order of operations:

      Please Excuse My Dear Aunt Sally

This sentence has words with begin with the same letters as the operations, in just the right order.  That is called a mnemonic device, which helps us to keep the information in our minds. 

Now, your problem has parentheses in it, and whatever is in parentheses must be done first.   All you need to do is add the numbers in parentheses and then multiply the result times two.  You will then have done your problem in the correct order of operations.