Answer
(a) Third harmonic
(b) $44cm$
Work Step by Step
(a) We know that the number of antinodes is equal to the number of the harmonic; hence, the given standing wave is a third harmonic.
(b) The required wavelength can be calculated as
$\lambda_3=\frac{2L}{n}$
We plug in the known values to obtain:
$\lambda_3=\frac{2(66cm)}{3}=44cm$