f(-x)=f(3-x)=f(x)
f(3-x)=f(x+3)=f(x-3)
f(x-3)=f(x+3)=f(x-3+6)
f(x)=f(x+6)
f(-5)=f(-5+6)=f(1)=-1
f(5)=f(-5)=1
f(5)+f(11)=f(5)+f(5+6)=2f(5)=2