Solve for t using natural logarithms. B=Pe^9rt

B = P. e^ (9rt)

ln (B) = ln (P. e^ (9rt)

ln (B) = ln (P) + lne^ (9rt)

lnB () = ln (P) + 9rt. lne

ln (B) = lnP () + 9rt

ln (B) - ln (P) = 9rt and t = [ln (B) - ln (P) ]/9r or t = [ln (B/P) ]/9r (both are correct)