Equation using 1, 5, 6, and 7 which equals 21

OK, I finally remember a neat puzzle presented by a college professor while I was still working at Raytheon.

What is the equation which equals 21 and only uses the numbers 1, 5, 6, and 7?

First Entry in the Daily Puzzle

I have no idea why I picked "The Daily Puzzle" for my blog other than I thought the concept was somewhat like "The Daily Cribbage". My first thought was to assemble a puzzle and document the progress on this blog. Oh the excitement! Another possibility would be to post a puzzle and welcome others to solve it. And yet another possibility would be to just let this blog die a slow death for lack of interest. But since this is the first entry lets hope for the best and may "The Daily Puzzle" have a long and interesting life.
Terry