When finding missing numbers in a problem which has parentheses you need to remember that the portion of the problem inside the parentheses always gets solved first. Here's an example:
3 * (2 + __) = 21 You know that 3 * 7 = 21 and so what you want is to figure out how the part of the problem inside the parentheses can become 7. Well, that's not too difficult since you know that 2 + 5 = 7. ANSWER: 5
Here's another example:
(4 + 2) * __ = 30 You know that the part of the problem inside the parentheses adds up to 6. So, the problem becomes 6 * __ = 30. The answer is clear at this point (providing that you know your basic facts). ANSWER: 5