At most, how many bright fringes can be formed on either side of the central bright fringe when light of wavelength 656 nm falls on a double slit whose slit separation is 3.45 × 10-6 m?

Answer 1

Pick the order m that has a diffraction angle of 90 degrees.

Use the formula at
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html#c1

Then double the order number, since there are fringes on both sides of the central bright fringe.