1. The problem statement, all variables and given/known data How exactly they combined equation1 and equation2 and got that system? I don't get that part. 2. Relevant equations A*(dy/dt)= -k*y eq1 A*(dz/dt)=ky-kz eq2 3. The attempt at a solution I tried substituting the 1st ky in the 2nd equation and then differentiating but I don't know how exactly they got that system which is: d2z/dt2 + 2* (k/A) *dz/dt +(k2/A2)*z =0 I did not understand how they got that derivation of the system by combining equation 1 and equation 2. I am willing to understand the concept, what are the steps, and the most important, how exactly to think to get that answer. Any hint and tips will be much appreciated. Thank you very much in advance.