Answer
Please see below.
Work Step by Step
Please note that a cosine function $y=A\cos Bx$ has amplitude $|A|$.
So the function $y=-3 \cos x$ has amplitude $|-3|=3$.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.