How do I find the domain and range of d?

The Domain is just the "x" values of your graph, if your x-axis is going toward both left and right forever, the domain should be all real numbers.

The Range, is just where the y value is, and because this graph has a minimum vertex value, the y should just be greater or equal to the y value of the vertex.

SO your graph should look like 

So we can see that our graph stretches to both left and right forever, so the domain is d=R, or all real numbers

The vertex is at a minimum value, so the range or y value is {y>=0.55}