Help plz I don't understand ​

From the top of a tree a bird looks down on a field mouse at an angle of depression of 50°. if the field mouse is 40 meters from the base of the tree, find the vertical distance from the ground to the bird's eyes.

What is the measure of angle a and angle c ?

The result of which expression will best estimate the actual product of (-4/5) x (3/5) x (-6/7) x (5/6) (i need an expression not an answer)