Normally, Lambda max refers to the wavelength in the absorption spectrum where the absorbance is maximum. The molecules absorb in a wavelength range centered around the lambda max. It acts as a single quantitative parameter to compare the absorption range of different molecules.
In your case if axis E represents the absorbance then your lambda max value is near 0.4 at A ≥ 800.
in addition to the absorbance spectrum also obtain the derivative spectrum (dA/dlambda vs lambda) of your samples, that will indicate the lambda max more clearly in spectrum( dA/dlambda = 0 at lambda=lambdamax or any stationary points of the spectral function).
First, you should carry our baseline correction. Second, there sould be carry our deconvolution, determining the number of bands. Third, a curve fitting procedure is needed in order to obtain the Lmax.