A crate slides down an incline right angled trough. The coefficient of kinetic friction between the crate and the trought is mew_k(not sure if i spelt it right). What is the acceleration of the crate in terms of mew_k theda and g?

I think I need to use F=ma and then to find a it would become a= F/m. What would I do next to solve this problem?

I will assume this is an inclined plane with angle theta with respect to the horizontal. I don't know what you mean by a "right angled trough"

You are correct that the equation to use is F = M/a, but the equation has to be applied along the direction of motion and F must be the net force, which is the difference between the Weight force component, M g sin theta, and the friction force, M g cos theta *(mew_k)

Therefore
a = g [sin theta - cos theta *(mew_k)]

Note than the mass M cancels out.