You need to substitute a value for symbolic variable t to get a numeric value
U = [3,2,1;
0,1,2;
0,0,1];
b = [0;0;0];
syms t;
x = backsub_syms(U,b);
y = subs(x, t, 1) % t = 1
double(y)
function x = backsub_syms(U,b)
n = length(b);
syms t
x=sym(zeros(n,1))
x(n)=sym('t')
b=(sym(b))
for i = n:-1:1
x(i)=b(i);
x(n)=sym('t')
if i<n
for j = n:-1:i+1
x(i)=x(i)-U(i,j)*x(j);
end
end
x(i)=x(i)/U(i,i);
end
end
