#### Answer

$5.1\times10^{9}$

#### Work Step by Step

To convert a number from standard notation to scientific notation, we must move the decimal to the left or to the right, counting the number of places, until the number is between $1$ and $10$, and multiply by a power of $10$. If the number is less than $1$, and we moved the decimal to the right, the power of $10$ is the negative number of places we moved the decimal point. If the number is greater than $1$, and we moved the decimal to the left, the power of $10$ is the positive number of places we moved the decimal point.
We are trying to convert $5,100,000,000$ to scientific notation.
We move the decimal $9$ places to the left, giving us $5.1$ as the coefficient, and $9$ as the power of $10.$
This gives us $5.1\times10^{9}$