lets suppose few apples = x
he further buy 12 more apples.
so equation would be
x + 12 (incomplete equation)
the he ate 2 apples,
x+12 - 2
the result is twice the apples he originally had, so
x + 12 -2 = 2x
x + 10 = 2x
2x - x = 10
x = 10

he originally bought 10 apples.