V= π·r²·h/3 is the formula to calculate this.
So, V= 3.14 · 7² · 21/3
Remember to use PEMDAS
Exponents are first so:
V= 3.14 · 49 · 21/3
Now division:
V= 3.14 · 49 · 7
Now multiply from left to right:
V= 153.86 · 7
V= 1077.02
But, in your case, the answer is D) 343π cm³