Description: Learn to solve the system of equations y = 4x - 17.5 and y + 2x = 6.5 using substitution. Created by Sal Khan and Monterey Institute for Technology and Education. We have this system of equations, y is equal to 4x minus 17.5, and y plus 2x is equal to 6.5. So this first equation is telling us, literally, by this constraint, y should be 4 times x minus 17.5. It also has to meet the constraint that it has to be 4 times x minus 17.5.*Teacher Advisor is 100% free.