Reimann i got it wrongand provide step by step clear explaination. i'd appreciate it.

Formula for Riemann Sum is:

interval is [1,3] so a = 1, b = 3
f(x) = 3x , sub into Riemann sum



Continue by simplifying using properties of summations.


Now you have an expression for the summation in terms of 'n'.

Next, take the limit as n-> infinity.
The limit of  goes to 0, therefore the limit of the summation is 12.

The area under the curve from [1,3] is equal to limit of summation which is 12.

Im sorry but what on