[د.امير]

Please look at the figure below
then
x=nλD/d, where n=1,2,3,4
for n=1
x1=5000E-10 m * 1m/0.002m= 2.5E-4m
for n=4
x4= 4*5000E-10 *1/0.002= 1E-3m
therefore, x4-x1= 1E-3 - 2.5 E-4 = 7.5E-4m