You have to make an assumption about how well the reservoir is "stirred", and the only one I can think of to make is that it is very well stirred, so the Al at the input is immediately at the output. Physically, though, this may not be likely.

Anyway, if you let x be the concentration of aluminum in the reservoir, then dx/dt will consist of two components - it will increase due to adding the aluminum, and decrease due to replacing the water at the current concentration with fresh water.

To solve equations like these, you need to separate the variables, so you'll have dx/(cx+d)=dt. Then integrate both sides and solve for x.

Post again in this thread if you're still having trouble.