[SOLVED] Integration of D.E.

Hi, having difficulty with this D.E. that relates to a tank of initially 400L of pure water, with 1L of salt water (concentration 4g/L) added per min, and 3L of the well-mixed tank solution removed per min. This is what I have so far;

I'm trying to find an integrating factor though I keep ending up with a final differentiated x(t) function that doesn't make sense (based on t-200, resulting in the function only existing from t > 200 when it should be from t > 0)

leading to

but then this doesn't seem to make the LHS equal to the derivative of ? Without the negative sign it would work fine but I'm sure that's the integral of P(t)... a little help? Maybe I've done something stupid...