If the region is , this is a distance of units.
Since you need subintervals, that means that each will be unit. This is going to be the length of each rectangle.
How do you know the width? You determine the co-ordinate and its corresponding . The co-ordinate represents the width.
So in your case:
If you are using the left-hand endpoints, then the first co-ordinate will be . So your co-ordinate is .
The second rectangle will be a unit more on the axis.
So and .
The next rectangle will be at and .
Once you have all the areas you need, you add them up.
For the right-hand estimate, you start at the right-hand endpoint and go to the left a unit each time.