From Förberedande kurs i matematik 1

Exactly as in exercise c, we write the left-hand side as \displaystyle 10^{-x}, and the right-hand side as \displaystyle 0\textrm{.}0001 = 10^{-4}, i.e. \displaystyle 10^{-x} = 10^{-4}, which gives \displaystyle x=4\,.