Answer
$(g\circ f)(0)=6$
Work Step by Step
The given functions:
$$f(x)=2x-3~~~~~~~~g(x)=-x+3$$
Composition of functions:
$$(g\circ f)(x)=g(f(x))=-(2x-3)+3=$$
$$=-2x+3+3=-2x+6$$
Substitute $x=0$ in composition:
$$(g\circ f)(0)=-2\cdot0+6=6$$