Question 972959
What is the reason for each of the grouped expression?  One polynomial is shown ADDED to another polynomial.  If no other reason needed for the grouping then the grouping is finished.

Now the indicated sum is  {{{3x^3+4x+14+(-4x^2)+21}}}
{{{3x^3+4x+14+(-4)x^2+21}}}
{{{3x^3+4x+14-4x^2+21}}}
and your son should know what to do next and finish.