I need help understanding what happened to the 2 in the numerator of the highlighted part of the problem in the screen grab please. Why not say 3^k+6^k/2? Thank you.

Answer:
Let's assume you went to last step, where:
3^k/2 + (2 x 3^k)/2
Denote 3^k = a, you have:
a/2 + (2 x a)/2 = (3 x a)/2
Substitute a = 3^k back, you have:
(3 x 3^k)/2
Hope it helps!
:)