Calculus for computers question

Hi, i have an assignment of 2 questions - both of which i cant do. Im asking for help for the first question to try and get the ball rolling as i have no idea...

We are given a formula, its just something random from a science book:

U(r) = A/r^12 - B/r^6

Where A and B are constants and r is the distance separating atoms (not important information). It doesnt say what U is, im assuming its like f(x) but U(r)...

i) Calculate r = rs, such that U(rs) is a minimum

(the "rs" is r(subscript)s).

Now, we are using the program mathematica to do this assignment, and i have attempted this question in a similar way to how the tut was done, that is to input the function and then find the 2nd derivative, which is:

156a/r^14 - 42b/r^8

and that is as far as i can get, i would appreciate any help!