Answer
$-2.059$
Work Step by Step
... Change of base: $\displaystyle \log_{a}x=\frac{\ln x}{\ln a}$.
$\displaystyle \log_{2.8}0.12=\frac{\ln 0.12}{\ln 2.8}=$ (calculator)
$\approx$-2.05926918318
rounded,
$-2.059$
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