You have the answer correct
The equation will be of the form:

where A is the amount after t hours, and r is the decay constant.
To find the value of r, we plug the given values into the equation, giving:

Rearranging and taking natural logs of both sides, we get:


The required model is:
-x^2+2x+3=x^2-2x+3 add x^2 to both sides
2x+3=2x^2-2x+3 subtract 2x from both sides
3=2x^2-4x+3 subtract 3 from both sides
2x^2-4x=0 factor
2x(x-2)=0
So x=0 and 2