Question 962539
Use the definition to find the result of x+3 as input of the function.  Examine the format of the general form expressions to determine k.


Definition:   {{{f(x)=x^2-2x-24}}}


Input (x+3):   {{{f(x+3)=(x+3)^2-2(x+3)-24}}}
{{{x^2+6x+9-2x-6-24}}}
{{{x^2+6x-2x+9-6-24}}}
{{{highlight_green(f(x+3)=x^2+4x-21)}}}


You were given that {{{highlight_green(f(x+3)=x^2+kx-21)}}}.
You can simply equate the two formulas for f(x+3).
{{{x^2+4x-21=x^2+kx-21}}}
{{{4x=kx}}}
{{{highlight(4=k)}}}