Joelle is working on a school report in a word processor. She wants to place an oversized image which is 30.51 \text{ cm}30.51 cm30, point, 51, start text, space, c, m, end text wide so that it appears centered on the page. The width of the page is 21.59 \text{ cm}21.59 cm21, point, 59, start text, space, c, m, end text. In the program, horizontal positions to the right of the leftmost edge of the page are positive. What horizontal position for the left edge of the image should Joelle use to center the image?