You can see there are three intervals and three right end points.
This can be best solved by creating a table like the following and in the end summing up the terms of summation:
k | ck | Δ xk | f(ck )*Δ xk
1 | 3 | 6 | 2* 3 *2 = 12
2 | 6 | 3 | 2* 6 *3 =36
3 | 8 | 2 | 2* 8 *2 = 32
V=12 + 36 + 32 = 80. (Ans)