ode3 := diff(P(t), t) = 0.2*P(t) - 300;
                        d                       
              

ICs := [[0, 1300], [0, 1800]];
DEplot(ode3, p(t), p = -50 .. 2000, t = 0 .. 2000, ICs, arrows = LINE, color = RED, linecolor = BLUE, title = "Solution Curves ode3");
Error, (in DEtools/DEplot) invalid variable in specification of independent variable range