Hello I need some help with this problem

For each natural number n, let fn be the nth Fibonacci number. Prove that (f_(n+1))^2 + (f_n)^2 = f_(2n+1).

so far what I have

let n=1 so 1^2 + 1^2 = 2 which is true so the equation holds for n= 1 (base case)

then I pick an arbitrary k where (f_(k+1))^2 + (f_k)^2 = f_(2k+1)

and now I prove for k+1 so I'm trying to show

(f_(k+2))^2 + (f_(k+1)^2 = f_(2k+3)

Ive spent hours trying to manipulate this problem to show this is true but no luck so far so I am hoping someone can guide me through this problem so I know how to do it thanks.