Answer
$\approx -3.880$
Work Step by Step
RECALL:
The change-of-base formula for logarithms:
$\log_a{b}=\dfrac{\log{b}}{\log{a}}$
Use a calculator and the formula above to obtain:
$\log_{1/3}{71} = \dfrac{\log{71}}{\log{(1/3)}} \approx -3.880$