How do i plot this quadratic graph?

I'm trying to plot this quadratic graph. For some reason it looks like this. It's not crossing the x axis for some reason the x axis are down. I need to fix it please.

https://imgur.com/a/FEFsG0B

This is the graph and the code.

[Setsuna Yuuki.]

that depends on the values of "a" "b" and "c", perhaps those values do not allow zero crossing.

[Yaseen Noaman]

They do. A is 1 b is -2 c is -4. They do cross the x axis

Answer 1

[Stephan]

x = -10:10;
y = polyval([1 -2 -4],x);
plot(x,y)
r = roots([1 -2 -4])
text(r(1),0,'\leftarrow Root')
text(r(2),0,'\leftarrow Root')