A 6ft tall guy walks directly away from a street lamp that is 18 ft above the ground at a rate of 4ft/s. Determine the following at the instant he is 24 ft from the base of the lamp.

a) the rate of increase in the length of his shadow.

b) the speed of the end of her shadow

I get 2ft/s for a) but i don't know if that's right, and for b) i don't know what to do.

Help please and thank you