SOLUTION: I totally don't understand how to start with this exercise. I am supposed to find the center and radius of a circle that is represented in this equation... (x-9)^2 + (y+1)^2

You can put this solution on YOUR website!
First thing you need to do is review the formula for a circle.
See this URL --> http://www.mathwarehouse.com/geometry/circle/equation-of-a-circle.php
Once you have read that, the equation you were given is in that form already.
So you just need to "pick the center and radius' directly from the equation
Center (H,K) = (9,-1)
radius r^2 = 49 so r = 7