Write your own implementation of polyval function and name it my_polyval
here is a simple example showiing how polyval function works -

evaluate the polynomial p(x)=3x²+2x+1 at the points 5,7,9 the polynomial coefficients can be represented by the vector [3 2 1].

p = [3 2 1];
x = [5 7 9];
y = polyval(p, x)
y = 1×3
86 162 262
my_polyval needs to be generic. you do not have to use varargin for implementing this, as the function always has two input arguments p and x which are vectors
function:
function y = my_polyval(p, x)
code to call function:
x = [5,7,9];
p1 = [3,2,1];
y1 = my_polyval(p1,x)
p2 = [4,3,2,1];
y2 = my_polyval(p2,x)