I am having trouble solving the question and don't know the steps or how to start. Can I get the answer in a written solution?

For what values of a does the graph of f(x) = -3x - 6 - 6 cos (x) have a horizontal tangent? O = (2n+ , )71 in = integer O x = (2n+ 7 )7+ in = integer O z = (2n+ = )7+ 6 in = integer O x = (2n+- 3 in = integer