\((m-\frac{1}{2})\lambda=a sin\lambda=a \frac{y_m}{L}\)

\(y_m=\frac{L}{a}(m-\frac{1}{2})\lambda\)

\(\Delta y=y_5-y_{-5}=\frac{[4.5-(-4.5)]L \lambda}{a}=\frac{9 L \lambda}{a}\)

\(\lambda=\frac{\Delta y a}{9L}=\frac{(34.73mm)(0.2mm)}{9(1.5m)}=5.145*10^{-7}m=515nm\)