[tex]$f(x)+f(x+1) \stackrel{1}{=} 3^x+3^{x+1} \stackrel{2}{=} 3^x+3^x\cdot3^1\stackrel{3}{=}3^x+3\cdot3^x\stackrel{4}{=}4\cdot3^x\stackrel{5}{=}4\cdot f(x)[/tex]
1) Substitution from definition.
2) We use [tex]$x^{a+b}=x^a\cdot x^b[/tex]
3) We are changing the order of 3 and [tex]3^x[/tex].
4) We add 1 and 3 [tex]3^x[/tex] and we get [tex]4\cdot 3^x[/tex]
5) Substitution from definition (but the other way than at the beginning).
