$AH=12\Rightarrow {{13}^{2}}={{12}^{2}}+{{X}^{2}}\Rightarrow 169=144+{{X}^{2}}\Rightarrow {{X}^{2}}=25\Rightarrow X=5\Rightarrow HD=5\,\,,\,\,HC=10\Rightarrow CD=10+5=15$