Splitting up the interval [0, 6] into 6 subintervals means we have
![[0,1]\cup[1,2]\cup[2,3]\cup\cdots\cup[5,6]](https://tex.z-dn.net/?f=%5B0%2C1%5D%5Ccup%5B1%2C2%5D%5Ccup%5B2%2C3%5D%5Ccup%5Ccdots%5Ccup%5B5%2C6%5D)
and the respective midpoints are

. We can write these sequentially as

where

.
So the integral is approximately

Recall that



so our sum becomes

4-2=2
6*2=12
2+12=14
14 is the answer
What you want to do here is plug 2 in for x. when a question asks you to evaluate a certain equation at a certain number, it's just asking you to plug the value in for your variable. i'm assuming that 12x + 8 is meant to be set over 4 as a fraction.
(12(2) + 8)/4 ... figure out the top first. 12*2 = 24, 24 + 8 = 32.
32/4 = 8, so your correct choice is D.
It would actually be -14 because 5+4-2=7 and if you multiply 7 by -2 you get -14