The number of significant figures is how many digits you want in the answer. Round to the correct number of digits.
To write 2.99792458x10^8 m/s with 3 sig.figs (7 rounds 9 up to 10)
3.00x10^8 m/s
To write 2.99792458x10^8 m/s with 5 sig.figs (2 rounds 9 down to 9)
2.9979x10^8 m/s
To write 2.99792458x10^8 m/s with 7 sig.figs (5 rounds 4 up to 5)
2.997925x10^8 m/s
