Answer
See below
Work Step by Step
a) The permutation of $r$ objects of the total is simply ordering these elements.
The number is expressed as,
$$P(n,r) = \frac{n!}{(n-r)!}$$
b) the answer is simply $P(15,3)$ as order does indeed matter,
$15!/12! = 15*14*13 = 2730$
