This uses the substitution method, followed by chain rule.

First substitute the bracket inside the cosine to something simpler. Remember to find the derivative of that substitution.

Afterwards, differentiate the equation with the substituted term, in terms of the new term to get the derivative in terms of the new term.

After that use chain rule to find the derivative of the whole equation at the start, and substitute back your X term. Simplifying it should give you the answer. hope it helps