1. The problem statement, all variables and given/known data The question is such : System is modeled as : y''+2y'+4y=u(t) find the time at which the system goes up 75,90,95 % of its final value. 2. Relevant equations 3. The attempt at a solution I have no idea how to touch that,I tried to find the source of the regular rise time (from 10 to 90%),and didn't got it.How the hell do I even start?It looks impossible to isolate expression for time,from PE solution . Also,from the answers in the book,the answers are approximations (t for 75% for example is ~0.9sec). Thanks in advance.