Answer
See answer below.
Work Step by Step
a) $Ca_3(PO_4)_2(s)+2\ H_2SO_4(aq) \rightarrow Ca(H_2PO_4)_2(aq)+2\ CaSO_4(s)$
Balance the phosporus first, then calcium, then sulfur.
b) $2\ NaBH_4(s)+H_2SO_4(aq)\rightarrow B_2H_6(g)+2\ H_2(g)+Na_2SO_4(aq)$
Balance the boron first, then sodium, then sulfur and hydrogen.
c) $WO_3(s)+3\ H_2(g)\rightarrow W(s)+3\ H_2O(l)$
d) $(NH_4)_2Cr_2O_7(s)\rightarrow N_2(g)+4\ H_2O(l)+Cr_2O_3(s)$
