Answer
$AD=13$
Work Step by Step
In $\triangle ABC$
$AC^{2}=AB^{2}+BC^{2}$
$AC^{2}=4^{2}+3^{2}$
so AC = 5
In $\triangle ACD$
$AD^{2}=AC^{2}+DC^{2}$
$AD^{2}=5^{2}+12^{2}$
$AD^{2}=25+144$
$AD^{2}=169$
$AD=13$
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