When you raise a coefficient/number to a negative power, you divide the number 1 by that coefficient and power, and make the power positive.
For example:
[tex] 3^{-2} [/tex] = [tex] \frac{1}{3^{2}}[/tex]
Vice versa:
[tex] \frac{1}{3^{-2}}[/tex] = [tex] 3^{2} [/tex]
With this in mind, plug in -3 for x.
[tex]y~=~6~*~4^{-3}[/tex]
[tex]y~=~6~*~ \frac{1}{4^{3}} [/tex]
[tex]y~=~6~*~ \frac{1}{64} [/tex]
[tex]y~=~ \frac{6}{64} [/tex]
[tex]y~=~ \frac{3}{32} [/tex]
or
y≈0.09735
