How do you make 8(.5^(n-1)) turn into 2^(4-n)?

Also what would -8(.5^(n-1)) turn into?

My guess for the first one is to multiply the equation by 2 to get 16x1^(n-1), therefore 16 would be 2^4. Then I get stuck

While the second one, it would be -2^(4-n)? Please explain to me, thank you.