7 9/4 write as expression

Bismuth-210 is an isotope that radioactively decays by about 13% each day, meaning 13% of the remaining bismuth-210 transforms into another atom (polonium-210 in this case) each day. if you begin with 233 mg of bismuth-210, how much remains after 8 days?

Can someone explain to me how to do

Of f(x) is byof f(x)=-3|x| x-.f(x)? a )f(x)=3|x| b) f(x)=|x+3| c) f(x)= -3|x| d) f(x)= -|x+3|