also here are my calculations for part (d)
for KBr the first two indices are ( 1 1 1 ), and (2 0 0 ), so h =1, k=1, l=1, and h=2, k=0, l=0, respectivley.
for the fist case d-hkl is 3.805 angstroms, and 3.295angstroms for the second.
plugging them into Braggs formula with the variable I gave...