To multiply two radicals together, we can simply multiply both the values inside the radicals together and put that in a radical.
So for 5*sqrt(45b) * 6*sqrt(48b), we get 30*sqrt(45b*48b) = 30*sqrt(2160b^2). This value can be simplified further by taking out b^2 and 12^2 from the radical giving us the answer 360b*sqrt(15).