Ok, here is my answer; however, I think it is wrong..
Let a=b=0
\(2f(0)=2(f(0))^2\), which implied: \(2(f(0))^2-2f(0)=0\) ---> \(2f(0)(f(0)-1)=0\); thus \(f(0)=0\) or \(f(0)=1\)
Now, consider a=1 and b=0
\(2f(1)=(f(1))^2+(f(0))^2\) Suppose \(f(0)=1\), then: \(w^2-2w+1=0--->w=1\) (Where \(w=f(1)\))
But f(0)=f(1)=1 then... (Well, I think the original function is one-to-one function). (Not sure how to prove it.)
So, I ruled out f(0)=1, so f(0)=0, and then f(1)=2
Then, I let a=2 and b=0, a=3 and b=0, etc..
Realized the following pattern:
f(0)=0
f(1)=2
f(2)=4
f(4)=8
f(5)=10
So I concluded (again not sure how to prove it) that \(f(x)=2x\), where \(x=a^2+b^2\)
So let x=25
\(f(25)=2*25=50\)
Now, since this is a linear function, only 1 possible value, which is 50 (So \(n=1\))
and well, since it is only 1 possible value, then \(s=50\)
so: \(n*s=1*50=50\).
Pretty sure I did a mistake somewhere, especially these "claims" without proof.