The wavelength #(lambda)# times frequency #(nu)# equals the speed of light #(c)#, which is #3.00xx10^8"m/s"# to three significant figures.
#c=lambda*nu#
#nu=c/lambda#
Frequency is measured in Hertz (Hz), which is #"/s"# or #"s"^(-1)"#.
Convert #"560 nm"# to meters since the unit for the speed of light is #"m/s"#.
#"1 nm"=1xx10^(-9)"m"#
#560 cancel"nm"xx(1xx10^(-9)"m")/(1cancel"nm")=5.6xx10^(-7)"m"#
#nu=c/lambda#
#nu=(3.00xx10^(8)color(white)(.)"m/s")/(5.6xx10^(-7)"m")=5.5xx10^(14) "s"^(-1)"##=##5.4xx10^(14)"Hz"#