Answer
$5040$
Work Step by Step
Arrangements $\Rightarrow$ Permutations. $P(n,r)=\displaystyle \frac{n!}{(n-r)!}$.
He takes r=7 from n=7 blocks, so
$ P(7,7)=\displaystyle \frac{7!}{(7-7)!}\qquad$ ... by definition of factorials, $0!=1$
$=7!$
$=7\times 6\times 5\times 4\times 3\times 2\times 1$
= $5040$