what is my mistake?

syms x
A=[x 2 1;1 0 -x;5 1 x];
x=-15:.1:15;
plot(x,det(A))
grid on

Answer 1

[Azzi Abdelmalek]

syms x
A=[x 2 1;1 0 -x;5 1 x];
x=-15:.1:15;
plot(x,subs(det(A),x))
grid on