no fancy math here, just some reasoning with words. I just feel like it
for one step in x-direction the value of the function doubles, so the common factor is 2.
so let's trace it back
(1,-6)
(0, -3)
For an exponential function the y-intercept is the "initial value".
do let's write is as a function
[tex]y = - 3 \times {2}^{x} [/tex]
you could also use the points and plug their numbers into the general formula, but I felt this explanation could give you more understanding, since it's meant to be more intuitive.
hope it helps
