What’s the answer? i don’t understand it!

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 answer this question? : )

Solve this equation using substitution. {4x + y = −2} {4x + 6y = 44}