Balls in a Bag

There are 10 coloured balls in a bag. 7 are red and 3 are blue. You remove one ball and, without replacing the first, then remove a second.

If the second ball is blue, the probability that the first ball was red can be written as $\frac{a}{b}$ where $a$ and $b$ are positive, co-prime integers. Calculate the value of $a + b$ .