Answer
$3$
Work Step by Step
We can locate the first diffraction minimum within the double-slit fringe pattern by dividing Eq. 36-23 by Eq. 36-22 and solving for m2. We obtain :
$m_2=d/a$
We are given that the separation is twice the slit width, so we obtain
$m_2=2$
We know the first minimum happens at $m_2=2$, so we have 1 bright fringe on each side of the central maxima. If we include the fringe at the center, we have the total of 3 bright fringes.
