Can anyone help me with this please

[mental steppe]

Yes any clue?

@lethal fern

Help

[balmy ravine]

@mental steppe Please don't ping individual helpers unless given permission.

[balmy ravine]

I would use the identity $\sin^2 x+\cos^2 x=1$ to express both $f_{4}x$ and $f_{6}x$ in terms of only $\sin x$. Then, matching coefficients should yield the required result.

[balmy robin]

Civil Service Pigeon