A woman stands at a horizontal distance $x$ from a mountain and measures the angle of elevation of the
mountaintop above the horizontal as $\theta$ . After walking a distance $d$ closer to the mountain on level ground, she finds the angle to be $\phi$ . Find a general equation for the height $y$ of the mountain in terms of $d, \phi,$ and $\theta,$ neglecting the height of her eyes above the ground.