Help with these two problems please

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?

Rewrite the following quadratic functions in intercept or factored form. show your work. y = 4x^2 - 19x- 5

Varies inversely with x. if y=8.5 when x=-1 find x when y=-1