finding point on a function

I have a function P using polyfit. How can I find the corrdinate on P when x=a?

Answer 1

[Ameer Hamza]

You can use polyval()

xv; % x data-points
yv; % y data-points
p = polyfit(xv, yv, 3);
x = a;
y = polyval(p, x);

Answer 2

[KSSV]

Read about interp1.

Let (x,y) be your data.

xi = a ; 
yi = interp1(x,y,xi) ;
[xi yi]