Question

The equation of the quadratic function represented by the graph y=a(x-3)-1 what is the value of a

Answer 1

The value of "a" can be anything you want it to be. Usually, it is chosen to make the function meet some additional constraint, such as going through a particular point. You have not provided any such constraint, so there is no rationale for choosing any particular value of "a".

Let a=2, if you like.