How fast is the height of the animal's shadow on the building decreasing when the dog is 5 m from the building

A spotlight on the ground shines on a wall 14m away. If a dog 0.5m tall , runs from the spot light towards a building at a speed of 1m/s, How fast is the height of the animal's shadow on the building decreasing when the dog is 5 m from the building

Let

x = distance of the dog from the wall at any time
y = height of dog's shadow on the wall at any time

By similar triangles,

(0.5)/(14 - x) = y/14

Rearranging the above function,

y = 7/(14 - x)

Differentiating (use the quotient rule and when simplifying, you will arrive at)

dy/dt = -7(dx/dt)/(14 - x)^2

and at x = 5,

dy/dt = -7(1)/(14 - 5)^2

dy/dt = -7/81

dy/dt = -0.086 m/sec.

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