reset
set border
set xtics 1
set ytics 20
set grid
set xzeroaxis lt 9 lw 2
set yzeroaxis lt 9 lw 2
set style line 1 lt -1 lw 3
set style line 2 lt 5 lw 3
set style line 3 lt 12 lw 3
set xrange [-4:9]
f(x)=(x+3)*(x-5)*(x-7)
plot f(x) ls 1