Answer
$12.6\cdot 10^{8}$ is not scientific notation because $N$ is greater than $10$. ($1\leq N<10$)
$4.8\cdot 10^{1.7}$ is not scientific notation because $m$ is written in decimal form, while it should be an integer.
$0.207\cdot 10^{-5}$ is not scientific notation because $N$ is less than $1$, while it should be at least $1$, but less than $10$.
Work Step by Step
Scientific notation for a number is an expression of the type
$N\cdot 10^{m}$,
where $N$ is at least $1$ but less than $10$ ($1\leq N<10$), $N$ is expressed in
decimal notation, and $m$ is an integer.
