Both are wrong.
Since Elisa is considered property but she missed negative sign at the end.
Property of logarithm formula: if and only if .
Matthew considered incorrect expression.
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if and only if ![\"\"](\"http://www.mathskey.com/test/fckeditor/editor/plugins/fckeditor_wiris/integration/showimage.php?formula=349523b86c1b445f05a8f5ae9ea036b4.png\")
.![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")