X1 X2 X3 X4

1.0 20 3.5 4

1.1 18 4.0 2

1.9 22 2.2 -

0.9 15 - -

My notes for this material a quite limited and not very intuitive, could someone explain to me how I can solve for the 4th value in X3? My notes make reference to Use linear regression, sweep left-to-right X3=a+b*X2+c*X1 and X4=d+e*X3+f*X2+g*X1 but I don't see where a,b,c,d,e,f and g are.

From my limited knowledge of regression there is generally a dependent variable y which we can predict B0 and B1 but this isn't the case here.

Can anyone point me in the right direction?