I mixed 10 mL of 9mM KI with 5 mL of 0.9mM in a flask, and then added it to a beaker that contained 10 mL of 45 mM KBrO3, and 10 mL of 90mM HCl. This reaction took 61.60 seconds to complete.

I think I need to find the change in concentration, but don't know how to do this. I'm stuck.

You need to correct this post. 0.9 mM of what?