Answer
17190 years ago.
Work Step by Step
Carbon-14 content reduces to $100\%\times\frac{1}{2}=50\%$ after 1 half-life. Carbon-14 content reduces to $50\%\times\frac{1}{2}=25\%$ after two half-lives. This reduces to $25\%\times\frac{1}{2}=12.50\%$ after 3 half-lives.
$\implies t=3\times t_{1/2}=3\times5730\,y=17,190\,y$
The mammoth lived 17190 years ago.
