Answer
$3-10^{\frac {3}{4}}=3-\sqrt [4] {1000} $
Work Step by Step
$\log \left( 3-x\right) =0.75\Rightarrow \log _{10}\left( 3-x\right) =0.75\Rightarrow 3-x=10^{0.75}\Rightarrow 3-x=10^{\frac {3}{4}}\Rightarrow x=3-10^{\frac {3}{4}}=3-\sqrt [4] {1000} $