for the third part solve the equation
tan 20=[30+(12t-5(t^2))]/16t
lets consider A as the origin, angle RAB basically means the angle that R's horizontal and vertical displacements make wrt A. so R's horizontal displacement is 16t and its vertical displacement wrt A is 30+12t-5(t^2), see the diagram attached to get a clearer idea of the explanation, p.s. sorry about the bad handwriting
in the diagram RAB is A, and y and x are the vertical and horizontal displacements respectively.