%interpolating polynomials%
x=[-12;2;3;9]
y=[-883;-226;6;-1]
n=length(x)
p=polyfit(x,y,n-1)
k=x(1)-(x(n)-x(1))/3:(x(n)-x(1))/1000:x(n)+(x(n)-x(1))/3
f=polyval(p,k)
plot(x,y,'o',k,f,'-')