l337 Converter - C#

Prompt the user to input a sentence. Once you have their sentence, you will convert their sentence to 'l337' (Links to an external site.). You will need to replace the letter on the left with the letter on the right, using the replace() function in the String class, from the following table: A @ E 3 H |-| S $ T 7 U |_|

Since we are very excited about our application, we want to make sure that every sentence ends with an exclamation mark! If it already ends with an exclamation mark, do not add another, otherwise concatenate a '!' at the end!

I've completed the top portion using the replace() function but I need help with the second part.