Daniel B. answered 02/23/22
A retired computer professional to teach math, physics
a)
For B to be orthogonally diagonalizable, it needs to be symmetric.
That condition provides all the missing entries
6 -2 -1
B = -2 6 -1
-1 -1 5
b)
There are many websites that give you the diagonalization
B = PDP-1
c)
Then you use the relation
Bn = PDnP-1
I am not sure what value of n you have in mind, but it works for any value.