The first step to answer this question is to ID which reaction types will occur.
Explanation:
Zn + #CuBr_2# -> will be a single replacement rxn
#H_2# + #Cl_2# -> will be a synthesis rxn
#MgCO_3# -> will be a decomposition rxn
KCl + #AgNO_3# -> this is a precipitation rxn
Zn + #CuBr_2# ->Cu + #ZnBr_2# (need to know the oxidation of zinc is +2 = memorize this!)
#H_2# + #Cl_2# -> HCl (need to know that H is +1 and Cl is -1)
#MgCO_3# -> MgO + #CO_2# (need to know that Mg is +2 and O is -2 and that carbonates produce #CO_2# and oxides when they decompose
KCl + #AgNO_3# -> #KNO_3# + AgCl (need to know that K is +1 and #NO_3# is -1)
Since both #K# & #NO_3# are in soluble compounds on both sides of this last equation, we call them spectator ions.
The net ionic equation for this reaction is: #Cl^-#(aq)#+Ag^+#(aq)#->AgCl#(s)
You should balance the equations as well!
Here is a video which discusses the topic of reaction prediction.
