we know that if f(a)=b then

and
if

then f(b)=a
so
since

therefor, f(-2)=0
we have 2 points, (-2,0) and (0,3)
we can use slope intercept form
y=mx+b
m=slope
b=y intercept
we have when x=0, y=3, that is the y intercept
y=mx+3
substitute the other point
(-2,0)
x=-2, y=0
0=-2m+3
-3=-2m
divide both sides by -2

the equation is

or
3x-2y=-6 in standard form