Find Missing No.

Since 7=2^3-1

26=3^3-1

63=4^3-1

124=5^3-1

215=6^3-1

342=7^3-1

therefore the missing number is 8^3-1=511

2^3 - 1 = 7

3^3 - 1 = 26

4^3 - 1 = 63

5^3 - 1 = 124

6^3 - 1 = 215

7^3 - 1 = 342

so 8^3 - 1 = 511

General term is $\boxed{n^{3}-1}$ $n=2\Rightarrow 7\\n=3\Rightarrow 26\\n=4\Rightarrow 63\\n=5\Rightarrow 124\\n=6\Rightarrow 215\\n=7\Rightarrow 342\\n=8\Rightarrow \boxed{\boxed{511}}$