The draws are independent, so
[tex]P(\text{red then blue})=P(\text{red})\cdot P(\text{blue})[/tex]
There is a total of 11 marbles in the bag, so
[tex]P(\text{red})=\dfrac3{11}[/tex]
and
[tex]P(\text{blue})=\dfrac5{11}[/tex]
So
[tex]P(\text{red then blue})=\dfrac{15}{121}[/tex]
