HELP ME PLEASE I WILL GIVE THE THING

How do you answer this question? (step by step)

Will give find an equivalent function to f(x)=5(2)^3x. the 5(2)^3x means 5(2) to the 3x power answer choices: a: f(x)=125^x(8)^x b: f(x)=10^2x c: f(x)=40^x d: f(x)=5(8)^x

Two cyclists 84 miles apart start riding toward each other at the same. one cycles 2 times as fast as the other. if they meet 4 hours later, what is the speed (in mi/h) of the faster cyclist?