Add $3.5\times 10^{-4}$ and $2.1\times 10^{-5}$, giving the answer in standard form.

Make the powers equal first: $2.1\times 10^{-5}=0.21\times 10^{-4}$. Then $3.5\times 10^{-4}+0.21\times 10^{-4}=3.71\times 10^{-4}$. Adding the decimal parts without matching the powers gives 5.6, and adding the exponents gives $10^{-9}$ — exponents are added when multiplying, never when adding.