A spotlight on the ground shines on a wall 200 meters away. A giant 20 meters tall walks from the spotlight to the wall, at a speed of 4 m./sec. ; his path is perpendicular to the wall. Let x be the distance from his feet to the spotlight and let h be the height of the shadow on the wall. Also let θ be the angle of elevation at the spotlight from the horizontal to the top of his head. (a) Draw a sketch of the problem, and find a formula relating h and x. (b) When the giant is 40 meters from the wall, find the height of the shadow and the rate of change of the height of the shadow. (c) What is the rate of change of θ at the time the giant is 40 meters from the wall?

So far, i think i got a good equation going on for this one. For part b particularly.

The equation i got involved similar triangles : h/(200-x) = 20/x, so i differentiated implicitly. However, im not stuck trying to find an h? When i substitute in the variables and have variables that are missing such as height and x, do i find their value at the instant that the giant is 40 meters from the wall (x=40)?

Overall , i'm kinda unsure if im using the right equations and the right numbers to substitute in my implicit derivative.