hello everyone!

the square of the mean of the consecutive integers 1,2,3,4,5 is 9

the mean of the squares of the same 5 numbers is 11

show algebraically that the square of any five consecutive integers is always 2 less than the mean of the squares of those 5 consecutive integers

Here's what I got, but I have a sneaky feeling I have done something wrong cos I ended up with a quadratic!!