Question 8264
It seems as though you are looking for an equation rather than an expression.

The equation that relates distance (d) to time (t) and rate of travel (r) is:

{{{d = rt}}} So, in the case of your dolphin, the rate of travel (r) is 8 km/h

So you (or Cindy) can write:

{{{d = (8 km/h)t}}} Now, if the dolphin travels for 3 hours, then t = 3 hrs and you can replace the t in your equation with 3 hrs.  So it would look like this:

{{{d = (8 km/h)(3 hrs)}}} and if Cindy can work out the arithmetic, she'll find that:

{{{d = 24 km}}} because 8 km/h times 3 hrs is 24 km.