To simplify the expression, we need to remember that the cosine function has the following identity:

Which gives an expression for the cosine of a sum of angles. In this case we have the following original expression:

If we compare this expression to the right side of the identity given above, we notice that we have a similar structure but, in this case, instead of alpha we have 3x and instead of beta we have x; for this reason, let:

Then we have, according to the identity given above:

Therefore, the expression given simplifies to:
