Question

a person is pulling a 100 lb box across a smooth horizontal surface. what is the acceleration of the box if they are pulling upward at an angle of 20° with a force of 75 lbs

Well, I know that there is no friction involved because it is a smooth surface and that n+TsinΘ-mg = 0
but I'm not sure why TcosΘ = m * asubX (means a with a lower exponent of x [or subscript])

I've been having trouble this whole semester but I'm starting to understand that I need to be clever and find values of variables in order to solve other variables, or substitute/manipulate one equation/solve for a variable algebraically in order to find another value of an equation. (I am horrible in algebra btw).