Answer
(a) $y = -sin(4x)$
(b) $y = cos(2x)$
(c) $y = 5~sin(3x)$
Work Step by Step
From the negative-angle identities:
$sin~(-\theta) = -sin~(\theta)$
$cos~(-\theta) = cos~(\theta)$
(a) $y = sin(-4x) = -sin(4x)$
(b) $y = cos(-2x) = cos(2x)$
(c) $y = -5~sin(-3x)$
$y = (-5)[-sin(3x)]$
$y = 5~sin(3x)$