n(n+1) + 2(n+1) over 2 = (n+1) (n+2) over 2

Assuming all of it is over 2 not just the "2(n+1) over 2"

(n+1) is common factor. so n(n+1) + 2(n+1) becomes

(n+1) (n+2)

Then just chuck it all over 2 to get

(n+1) (n+2) over 2

Ahh, thank you both, that makes sense now.