To find the dollar amount Marcelo still needs to save after working h hours, we can use the following expression:
Amount still needed = Total cost of the bike - (Earnings per hour * Number of hours worked)
In this case, Marcelo wants to save $300 for a new bike, and he earns $9 per hour at his job. So the expression becomes:
Amount still needed = $300 - ($9 * h)
Let's say Marcelo has already worked for 5 hours. To find out how much more he needs to save, we can substitute h with 5 in the expression:
Amount still needed = $300 - ($9 * 5)
Amount still needed = $300 - $45
Amount still needed = $255
Therefore, Marcelo still needs to save $255 after working for 5 hours