1. (5t^3)^-4
=1/(5t^3)^4
=1/625t^12
2. 2x10^4=2x10000=20000
1x10^4=1x10000=10000
7x10^-9=7/10^9=7/1000000000=0.000000007
4x10^-7=4/10^7=4/10000000=0.0000004
By increasing order=7x10^-9,4x10^-7,1x10^4,2x10^4
3. (8x10^4)(9x10^-8)
We will multiply 8 by 9 and since the base(number) is the same-10, we add the powers together and then multiply that term with 72.
72x10^(4+-8)
=72x10^-4
=72/10^4
=72/10000
=0.0072
=7.2 x 10^-3
I hope this answer helps.