Confused about a simplification step in induction

Question

Hello - I don't know how they got from the 3rd line to the 4th line. I understand all other parts of the simplification.

Answer 1

Notice, from third line to reach fourth line/answer as follows $$8^{n+1}-8^n-14$$ re-writing $8^{n+1}=8\cdot 8^n$ & $8^{n}=1\cdot 8^n$,

$$=\underbrace{8\cdot 8^{n}-1\cdot 8^n}-14$$ factoring out $8^{n}$, $$=(8-1)8^{n}-14$$ $$=7\cdot 8^{n}-14$$