Answer:

Explanation:
Use the Product Rule for derivatives, which states that:
In the function we are given,
, we can break it up into two factors: -x is being multiplied by cos3x.
Now, we have the factors:
Before using the product rule, let's find the derivative of cos3x using the chain rule and the power rule.
Now let's apply the product rule to f(x) = -xcos3x.
Simplify this equation.
Multiply x and 3 together and remove the parentheses.
Therefore, this is the derivative of the function
.