Answer:
• From wave equation :
[tex]{ \rm{v = f \lambda}}[/tex]
[tex]{ \rm{3.0 \times {10}^{8} = (1.89 \times {10}^{18}) \times \lambda}} \\ \\ { \rm{ \lambda = \frac{3.0 \times {10}^{8} }{1.89 \times {10}^{18} } }} \\ \\ { \boxed{ \rm{ \: \lambda = 1.587 \times {10}^{ - 10} \: }}}[/tex]