#### Answer

(a) F=kx
(b) k=7.5
(c) F=45 N

#### Work Step by Step

(a) Hooke's Law states that:
$F=kx$
where $F$ is force and $x$ is distance stretched.
(b) We are given $F=30$ and $x=9-5=4$ and can use this to find $k$:
$30=k*4$
$k=30/4=7.5$
(c) We are given $x=11-5=6$ and need to find $F$:
$F=7.5*6=45\ N$