Answer:
[tex]Solution,\\f(x+3)=f(x)+f(3).......(1)\\Put~x=0~in~eq^n~(1),\\f(0+3)=f(0)+f(3)\\or, f(3)=f(0)+f(3)\\or, f(0)=0.........(2)\\Now,\\Put~x=-3~in~eq^n(1),\\f(-3+3)=f(-3)+f(3)\\or, f(0)=f(-3)+f(3)\\From~eq^n(2),\\0 = f(-3)+f(3)\\or, f(-3)=-f(3)~proved[/tex]
