When rearranging this formula involving a square root on the right side, can someone please explain to me where the +6m in the second step comes from when squaring the left side? I'm going through my revision guide and I don't feel this has been explained at all, although I am probably missing something simple.
Please excuse the crudely put together image taken from the guide as I don't know how to write this formula as text.
Please excuse the crudely put together image taken from the guide as I don't know how to write this formula as text.