Explanation:
why are you panicking ? there is no reason for panic. just take a deep breath and then use common sense and basic logic.
what do you see ?
you see 1 can. in a 1×1 configuration.
you see 4 cans in a 2×2 configuration.
you see 9 cans in a 3×3 configuration.
how will it continue ? what do you think ?
and what is that configuration shape ? hmmm ?
it is a square, plain and simple : the same number of units for the length and the width.
and since every layer is then clearly adding 1 can in every direction, it continues with 4×4, 5×5, 6×6, ...
and the 9th layer is ... hmm ? 9×9 ! Tatah !
and so, the number of cans in every layer are the sequence of square numbers : 1 (1²), 4 (2²), 9 (3²), 16 (4²), ...
and the 9th layer has then 9² (or 9×9) = 81 cans.
you really think this was difficult ?
I cannot believe that.
just take another look without any panic and think it through.