Can you tell me what page the following quote can be found on? "As though if the full force of it were unstoppable it would roll like thunder."

“It was as though only a fraction of that voice were being used, as though if the full force of it were unstopped it would roll like thunder.”

This statement appears about midway through chapter 4 in Elizabeth George Speare's The Bronze Bow. The exact page number may not be the best way to locate the quote, because in different editions, the quote may appear on different pages.
However, don’t worry! You can still find the quote easily, without having to do much rereading.
Open to chapter 4, and then flip to about halfway through that chapter. Then start scanning the text, looking for the paragraph that starts with the sentence “A shock ran through Daniel at the first words.” Skip to the end of that paragraph, and you’ll find the quote in question.
Notice what page this quote appears on in your particular edition of the book—that’s the page number you’ll use for any citations, and your citation sheet will include information about the particular edition of your book.
In general, this is a great skill to learn: how to find, quickly, the context of an important quote.
For works that are in the public domain (which generally means that the entire text can be found online for free), this process can be even easier. You can go to a search engine and type in a few key words from the quote—words that you think are unusual enough all together in a string that they would be unlikely to appear again in another part of the text. Or you can simple search for a full PDF of the text. Once you have found an electronic copy of the text, you're good to go: use the browser’s “Search” or “Find” function to jump instantly to the quote you need. This makes it easy to search for particular mentions of characters, particular quotes and key terms, and so on.