Call

Balanced equation

$"4Fe(s) + 3O"_2("g")"$$rarr$$"2Fe"_2"O"_3("s")"$

Multiply the given moles of iron by ratio between oxygen and iron from the balanced equation, with moles oxygen in the numerator.

$6.4color(red)cancel(color(black)("mol Fe"))xx(3"mol O"_2)/(4color(red)cancel(color(black)("mol Fe")))="4.8 mol O"_2"$