Recurring relations solving differential equations

[Huumah]

Can anyone explain why we obtain the part where i put the red underline.
I understand everything until then

 

[LCKurtz]

In the line above your red line, write the first sum as the two sums$$
\left(\sum_{n=0}^1 + \sum_{n=2}^\infty\right)(n+2)(n+1)a_{n+2}x^n$$
The red lined terms come from the first sum. The second sum starting with $n=2$ is combined with the other like sum.