x=[2 4 6 8];
n=length(x);
y=[2 11 28 48];
x1=sum(x);
x2=sum(y);
x3=sum(x.^2);
x4=sum(x.*y);
xi=[n x1;x1 x3];
yi=[x2;x4];
a=xi\yi;
xx=2:0.001:8;
yy=7.75.*xx-16.5;
yy1=6.55.*xx-12.5;
plot(x,y,'r',xx,yy,'y',xx,yy1)