Answer
$6.8^{\circ}$
Work Step by Step
First, we can use the given frequency, $f=25*10^3Hz$, and the speed of sound in water, $v=1450m/s$, to find the wavelength.
$\lambda = v / f$
$\lambda = (1450m/s) / (25*10^3Hz)$
$\lambda =0.058m$
Now we can use the given diameter, $d=0.60m$, in equation (36-12), to find the angle to the first minimum, $\theta$.
$\sin \theta= 1.22 \frac{\lambda}{d}$
$ \theta=\arcsin( 1.22 \frac{\lambda}{d})$
$ \theta=\arcsin( 1.22 \frac{0.058m}{0.60m})$
$\theta = 6.8^{\circ}$
