.
In this problem, they expect that you will use this standard Trigonometry identity
sin(a)*cos(b) - cos(a)*sin(b) = sin(a+b),
which is valid for any/all angles " a " and " b ".
Then the solution is in one line, as it is shown below.
Let a = 2x, b= 4x. Then sin(2x)*cos(4x) − sin(4x)*cos(2x) = sin(2x-4x) = sin(-2x) = -sin(2x).
Solved and completed.
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The method shown in my post, is a traditional
and standard method solving similar problems.
If you will solve it as @MathLover1 does, your teacher will learn
that you do not know a standard way solving such problems.