Description: Learn to solve the system of equations y = (-1/4)x + 100 and y = (-1/4)x + 120 using substitution. Created by Sal Khan and Monterey Institute for Technology and Education. Or another way to think about it is, if y is equal to this whole thing right over here and if we have to find an x and a y that satisfy both of these equations, if y is equal to that, why can't I just substitute that right here for y? And if we do that, the left-hand side of this bottom equation becomes negative 1/4x plus 100. And then that is going to be equal to this right-hand side is equal to negative 1/4x plus 120.*Teacher Advisor is 100% free.