Answer
$B = 3.3~T$
Work Step by Step
We can convert the angular speed to units of rad/s:
$\omega = 350~rpm \times \frac{1~min}{60~s} \times \frac{2\pi~rad}{1~rev} = 36.65~rad/s$
We can find the strength of the magnetic field:
$emf = \omega~NBA$
$B = \frac{emf}{\omega~NA}$
$B = \frac{emf}{\omega~N~\pi~r^2}$
$B = \frac{17.0~V}{(36.65~rad/s)(50)(\pi)(0.030~m)^2}$
$B = 3.3~T$
