Answer
$1.5\;\rm m$
Work Step by Step
We know that the time between two successive instants takes for the two antinodes to be at maximum height is half the period, see your textbook Figure 21.4.b.
Hence,
$$T=2\Delta t=2\times 0.25=\bf 0.5\;\rm s$$
We know that two waves are having the same wavelength, and the wavelength is given by
$$v=\lambda f$$
where $f=1/T$,
$$v=\dfrac{\lambda }{T}$$
Hence,
$$\lambda=vT=3\times 0.5$$
$$\lambda=\color{red}{\bf 1.5}\;\rm m$$
