Dave jumps off a bridge with a bungee cord( a stretchable cord) tied around his ankle. He falls for 15m before the bungee cord begins to stretch. Dave's mass is 75kg and we assume the cord obeys Hook's law with elastic constant, k= 50N/m. if we neglect air resistance and the mass of the cord, calculate how far below the bridge Dave will fall before coming to a stop.

i understand that i have to use the formula F=Ke and i need to find F.

how should i go about finding it? which formula to use?