prob=Probability[x≥a\[Conditioned]x≥300,x\[Distributed]WeibullDistribution[2,1000],Assumptions->a>0];

Labeled[Plot[prob,{a,0,1700},PlotRange->{0,1},Filling->Axis,PlotStyle->Thick,FillingStyle->Hue[.1,.8,.9],ImageSize->400],Style[Row[{"Pr[Life ≥ a \[Conditioned] Life 
≥ 300] = ",prob}],FontFamily->"Verdana"]]