The main idea is the number of marbles and the denominator of the probability. All you have to do is multiply the fraction by a fraction which is equal to 1 which makes the denominator equal to 63. We know 63 is a multiple of 9. 9 goes 7 times in 63 so we multiple the fraction with
which gives,
which when evaluated gives
and the numerator is the number of red marbles in the bag. So there are 14 red marbles in the bag :D