Question: we illuminate two long parallel slits that each have a...

We illuminate two long parallel slits that each have a width b = 10 μm with plane waves of light with a wavelength of λ = 500 nm. The fringe pattern that falls on an observation screen far away is comprised of 11 maxima that decrease in brightness on either side of the central peak, calculate what the distance, a, between the slits

My attempt at this question was using the equation $a\sin {\theta }_m=m\lambda$, thus:

$a=\frac{m\lambda }{\sin {\theta }_m}a=\frac{1*500*{10}^{-6}}{\sin {\theta }_1}$

From there I wasn't sure how to find $\sin \theta$

Thank you