Let's see
[tex]\\ \rm\rightarrowtail E=hv[/tex]
[tex]\\ \rm\rightarrowtail E=\dfrac{hc}{\lambda}[/tex]
[tex]\\ \rm\rightarrowtail \lambda=\dfrac{hc}{E}[/tex]
[tex]\\ \rm\rightarrowtail \lambda=\dfrac{6.626\times 10^{-34}\times 3\times 10^8}{2.89\times 10^{-19}}[/tex]
[tex]\\ \rm\rightarrowtail \lambda=6.878\times 10^{-7}m[/tex]
[tex]\\ \rm\rightarrowtail \lambda=68.8\mu m[/tex]