Answer
(a) $-0.54mm$
(b) $0.3mm$
Work Step by Step
(a) We know that
$\Delta L=L\alpha \Delta T$
We plug in the known values to obtain:
$\Delta L=12\times 10^{-6}(-45)$
This simplifies to:
$\Delta L=-0.54mm$
(b) As $\Delta L=L\alpha \Delta T$
We plug in the known values to obtain:
$\Delta L=12\times 10^{-6} \times 25$
This simplifies to:
$\Delta L= 0.3mm$