Question 293645
{{{f(x)=x^2+2x}}} Start with the given function.



{{{f(a+3)=(a+3)^2+2(a+3)}}} Plug in {{{x=a+3}}}



{{{f(a+3)=a^2+6a+9+2(a+3)}}} FOIL



{{{f(a+3)=a^2+6a+9+2a+6}}} Distribute.



{{{f(a+3)=a^2+8a+15}}} Combine like terms.