The speed of light is now to be defined 2.99792458x10^8 m/s express the speed of light to three significant figures five significant figures seven significant figures

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