How do I know the rate law predicted by a mechanism if the 2nd of 3 elementry steps is the rate limiting step?

The reaction rate is determined by the slowest step (aka the rate limiting step). Think of it this way, you have 3 girls baking cookies. Girl #1 can mix the dough quick enough to make 20 brownie parts/hour. Girl #2 is a bit slow and can only put 5 brownies on a pan/hour. Girl #3 is also pretty quick and can heat up 40 brownies/hour. But since Girl #2 can only put 5 brownies on a pan in one hour. Girl #3 can only heat up 5 brownies/hour. Ha. I never thought the day would come in which I would explain rate determining steps in terms of baking brownies.