GIVEN :-

Lamda = 1.54 Angstrom = 1.54 x 10^-10 m.
Theta = 22.4 degrees.
Spacing of (2,0,0) planes in Aluminium = d = [a / square root of ( 2² + 0 + 0 )] i.e a / 2.
a = ?

      2d sin(theta) = n Lamda
i.e.       2 x (a/2) x sin(22.4) = 1 x 1.54 x 10^-10
i.e.       a = (1.54 x 10^-10) / (0.3811)
i.e.       a = 4.04 A⁰