The reaction is a synthesis…
[tex]4Al + 3O_2 \longrightarrow 2Al_2O_3\\[/tex]
… because two substances are combining to make one other substance.
[tex]4\stackrel{\hbox{0}}{\hbox{Al}} + 3\stackrel{\hbox{0}}{\hbox{O}}_2 \longrightarrow 2\stackrel{\hbox{+3}}{\hbox{Al}}_2\stackrel{\hbox{-2}}{\hbox{O}}_3\\[/tex]
It is also a reduction-oxidation reaction because the oxidation number of Al increases from 0 to +3 and the oxidation number of O decreases from 0 to -2.
