It is the method by which the value of a variable in equation 1 is substituted in another equation and the value of the other variable in equation 2 is substituted in equation 1 or equation 3 made by equation 1.Let\’s understand it with an example:x+y=2…………. eq 1x-2y=1…………. eq 2In equation 1, x+y=2x=y-2…………. eq 3substituting the value of x in equation 2x-y=1(y-2) -2y=1y-2-2y=1-y-2=1-y=1+2-y=3[y= -3]Substituting the value of y in equation 3x=y-2x=-3-2[x=-5]Hope you understood.