I'm not quite sure how to solve this problem. I tried one step but don't know what to do next. It seems weird.

dw/dt=-lambdaW(t) with W(0)=Winitial

All I did was separate:

dw/-lambdaW(t)=dt but don't now what to do from here because I don't know what W(t) is.

Can anyone help? I will deeply appreciate it! thank you!