$f=5.25times 10^{14}$ Hz.

Wavelength $lambda$ is given by

$$
lambda=frac{c}{f}=frac{(3times 10^8 m/s)}{(5.25times 10^{14} Hz)}=5.71times 10^{-7} m=571 nm
$$

$$
lambda=620 {rm nm}=620times10^{-9} {rm m}
$$

$$
h=1.8 {rm m}
$$

Unknown:

number of wavelength $n=?$

$$
n=frac{h}{lambda}=frac{left(1.8 {rm m}right)}{left(620times10^{-9} {rm m}right)}=2.90times10^{6}
$$

$$
lambda=0.30 {rm nm}=0.30times10^{-9} {rm m}
$$

$$
c=3times10^{8} {rm m/s}
$$

Unknown:

$$
f=?
$$

We know

$$
f=frac{c}{lambda}=frac{left(3times10^{8} {rm m/s}right)}{left(0.30times10^{-9} {rm m}right)}=1.0times10^{18} {rm Hz}
$$