Question 388983
{{{f(n) = n^2 + 3n = 28}}}, or {{{n^2 + 3n - 28 = 0}}}.
(n + 7)(n - 4) = 0.  Then n = -7 or n = 4.  These give g(-7) = 2*-7 = -14, or g(4) = 2*4 = 8.