Explanation:
Simplifying
f(x) = 2cos(3x)
Multiply f * x
fx = 2cos(3x)
Remove parenthesis around (3x)
fx = 2cos * 3x
Reorder the terms for easier multiplication:
fx = 2 * 3cos * x
Multiply 2 * 3
fx = 6cos * x
Multiply cos * x
fx = 6cosx
Solving
fx = 6cosx
Solving for variable 'f'.
Move all terms containing f to the left, all other terms to the right.
Divide each side by 'x'.
f = 6cos
Simplifying
f = 6cos