Subtract $4\times 10^{-6}$ from $9\times 10^{-5}$, giving the answer in standard form.

Write both with the same power: $4\times 10^{-6}=0.4\times 10^{-5}$. Then $9\times 10^{-5}-0.4\times 10^{-5}=8.6\times 10^{-5}$. Doing $9-4$ straight off gives the trap $5\times 10^{-5}$, which ignores that the two powers of ten differ; adding rather than subtracting gives $9.4\times 10^{-5}$.