Total number of balls =(9+12)=21.
Let S be the sample space. Then,
n(S) = number of ways of selecting 1 ball out of 21 = 21.
Let E be the event of drawing a red ball. Then,
n(E) = number of ways of selecting 1 red ball out of 9 = 9.
∴
P(getting a red ball) =P(E)=
n(E)
n(S)
=
9
21
=
3
7
.
