Well, what have you tried to do yourself?
If you divide the interval from 0 to b into n equal length subintervals, what is the length of each subinterval? If you take to be the right hand endpoint of each subinterval, what is in terms of i? Then is the height of each rectangle what the length of the subinterval is the base. The area of each rectangle is the product of those two. Find the entire area by summing those. With any luck, you will get a sum that looks like , perhaps multiplied by other numbers.