you can arrange the 7 white balls in 7! ways as all balls are identical so it should be (7!/7!)
now for black balls you have 8 places. from 8 places you can choose any of the 3 places= 8C3
and you can arrange them in 3! ways as all the 3 balls are identical so it should be (3!/3!)
hence solution should be (7!/7!)*8C3 *(3!/3!) = 56