How does this seemingly-trivial simplification work?

In a section on inductive proofs in the book Modelling Computing Systems: Mathematics for Computer Science (Muller, Struth) there is a simplification that is assumed to be trivial, but that I can't figure out.

It occurs in this step:

$\frac{k(k+1)(k+2)}{3}+(k+1)(k+2)\stackrel{?}{=}\frac{(k+1)(k+2)(k+3)}{3}$

How does one get from the first expression to the second?

In a section on inductive proofs in the book Modelling Computing Systems: Mathematics for Computer Science (Muller, Struth) there is a simplification that is assumed to be trivial, but that I can't figure out.

It occurs in this step:

$\frac{k(k+1)(k+2)}{3}+(k+1)(k+2)\stackrel{?}{=}\frac{(k+1)(k+2)(k+3)}{3}$

How does one get from the first expression to the second?