reset
set border
set xtics 0.5
set ytics 5
set grid
set xzeroaxis lt 2 lw 2
set yzeroaxis lt 2 lw 2
set style line 1 lt 3 lw 3
set style line 2 lt 5 lw 3
set style line 3 lt 12 lw 3
set xrange [-7:4]
set yrange [-20:25]
f(x)=x-6
g(x)=x**2+3*x-18
h(x)=f(x)/g(x)
k(x)=-x**2-3*x+18
plot h(x) ls 1,k(x) ls 2