Can someone explain parts a and b on this problem?

30. Let L be the length of the rope. X ROW (d) L2 = 144 + x2 w 809 TP - (b) If - = -4, and L = 13, 21 UL dx dt .0 = 2x 2 dt dt dL x dx dx = L dL 4L dL since = -4 ft/ sec. dt L dt x dt x dt anges three times as 5 = When L = 13, 13 (-4) One-SUIT 4 fu/sec or am Lay chirag ile of the x = VL2 - 144 = V169 - 144 = 5 13 ft -20 = ft/sec 13 dx 4(13) 52 = = - 10.4 ft/sec. dL at 5 5 As L - 0, increases. dt Speed of the boat increases as it approaches the dock