How does this substitution rule in calculus work?
how can we use the substitution rule here to get u = ln^3 y
and du = ((3ln^2 y)/y) dy
so how do we get from the first equation to:

Darcy deposited $390 into a savings account. as long as the account balance remains at or above $150, she does not have to pay a service fee for the account. for how many months can she withdraw $40 a month without having to pay service fees? a) 3 b) 5 c) 6 d) 9