#### Answer

$-5.04 \times 10^{-9}$

#### Work Step by Step

To write the number in scientific notation, do the following steps:
(1) Move the decimal point 9 places to the right to have $-5.04$;
(2) Multiply $10^{-9}$ to retain the value of the original number and have: $=-5.04 \times 10^{-9}$