You have an opening parenthesis there that doesn't have a closing one. Where is the closing parenthesis supposed to go?
Here's a little algorithm you can use to check to make sure all opening ('s have closing )'s: scan the expression from left to right. Every time you have a left (, increment a counter. Every time you have a right ), decrement the counter. If, at the end, you have 0, then your expression is at least balanced. Then you also have to make sure you don't have anything like ([)], which is not allowed. It must always be [()] or ().
Be careful with parentheses!