The concept of Linear combination is simple that you already learnt to find the 'X' value from the equation. ex. y = aX +b When you replace 'b' with 'X2' then it will be: y = aX + X2 When you add another constant value for 'X2' with c then: y = aX + cX2 Finally you have two constant values 'a' and 'c' which weight to 'X' and 'X2'. Hence Linear combination is you multiply the constant values ('a', 'c') with Xs and you sum them. Like below. x(t) = ax1(t) + bx2(t) To know more Vector combination with this Click .