Imagine the slide as the hypotenuse of a right triangle. If you pretend, at first, that it's sitting on the ground, the slide and the 'ground' form a 50° angle. Draw the diagram. Label all the parts you know. Add the 6' drop. Where are you standing in the diagram?
Is the height of the slide the side opposite 50° or adjacent to 50°? Remember to add 6' to your answer.