Basic Rules of Differentiation: Videos & Practice Problems

A pebble is dropped from a bridge, and the distance it falls is given by the equation $d\left(t\right)=15t^2$, where $d$ is in meters and $t$ is in seconds. If the pebble takes $7$ seconds to reach the water below, what is the height of the bridge? Also, calculate the pebble's speed upon hitting the water in kilometers per hour.