Question 707866
Here is how to find your way:


replace the variable, x, with the number t+1, and replace the variable, x, with the number t-1.  Evaluate the function for each case.  The key idea is to use substitution.  


{{{f(t+1)=(3(t+1)^2)+5}}}
={{{3(t^2+2t+1)+5}}}
={{{3t^2+6t+3+5}}}
={{{3t^2+6t+8}}}, and maybe you complete the square to get standard form if you wish to.
That was for x=t+1.


Now do the same using t-1 in place of x.
{{{f(t-1)=(3(t-1)^2)+5}}},
...and you know what to do...