The shadow casts by a man standing 1 m from a lamppost is 1.2 m long. If the man is 1.8 m tall and walks away from the lamppost at a speed of 120 m/min, at what rate is the shadow lengthening after 5 seconds?

This is my work:

I used Pythagorean theorm

a^2 + b^2 = c^2

differentiating

2a da/dt + 2b db/dt = 2c dc/dt

2(1) da/dt + 2(1.8) db/dt = 2(1/12* -120) dc/dt