Step-by-step explanation:
Given that, the height of the tide measured at a seaside community varies according to the number of hours t after midnight. The height is given by the equation as :
![h=-(1)/(2)t^2+6t-9](https://img.qammunity.org/2020/formulas/physics/high-school/li6ri4s8evd10pk5iqdlyqrzbw2emrj3x1.png)
When the tide first be at 6 ft, put h = 6 ft in above equation as :
![-(1)/(2)t^2+6t-9=6](https://img.qammunity.org/2020/formulas/physics/high-school/oknqa58ezcocxkupbty66hbtcuo6gsio44.png)
![-t^2+12t-18=0](https://img.qammunity.org/2020/formulas/physics/high-school/wz2n4cx1tmel4siwvpu2qaa6xcrlk02b3m.png)
On solving the above equation to find the value of t. It is equal to :
t = 3.551 seconds
or
t = 8.449 seconds
So, the tide of 6 ft is at 3.551 seconds and 8.449 seconds. Hence, this is the required solution.