the given series is -9, -14, -19, -24
first term is = -9
second term is = -14
the common difference is, d = (-14-(-9))
d = -14 + 9 = -5
the third term is -19
now the common difference is
d = (-19- (-14))
d = -19 + 14
d = -5
here the common difference is constant, so the given series is in the arithmetic series.