We can figure this out using the explicit formula.

n represents the term we are looking for.
f(1) represents the first term in the sequence, which in this case, is 7.
d represents the common difference, which in this case, is +3.
f(n) = 7 + 3(n - 1)
f(n) = 7 + 3n - 3
f(n) = 4 + 3n
Now, we can input 214 for n and solve.
f(214) = 4 + 3(214)
f(214) = 4 + 642
f(214) = 646
The 214th term in this sequence is 646.
A triangular prism Vol. = area (A) of the triangular base × height (h) of prism
A triangular pyramid V = A × 1/3 h
So since they have same base A and h, the V of the pyramid will be exactly 1/3 the V of the prism