Answer
(a) $56cm$
(b) $6.5cm$
Work Step by Step
(a) We know that the horizontal distance from crest to trough is known as half of the wavelength.
Thus $\frac{\lambda}{2}=28cm$
$\implies \lambda=2\times 28cm=56cm$
(b) We know that the maximum vertical displacement from the mean position is known as amplitude and the total distance from crest to trough is twice the amplitude.
Thus $2A=13cm$
$\implies A=\frac{13}{2}cm=6.5cm$