Here is the problem:

As you know, when a course ends, students start to forget the material they have learned. One model (called the Ebbinghaus model) assumes that the rate at which a student forgets material is proportional to the difference between the material currently remembered and some positive constant, .A.Let be the fraction of the original material remembered weeks after the course has ended. Set up a differential equation for , using as any constant of proportionality you may need (let ). Your equation will contain two constants; the constant (also positive) is less than for all .

I've found the equation is k(a-y)

but it also asks what the initial condition of the equation is?

and I cannot figure this part out.