How do you solve the system #2x + y = 1# and #4x + 2y = −1# by substitution?
We would normally rearrange  to get an equation for
and then substitute this value into equation  to solve for
Notice something unusual in this case
The left-side of  is exactly 2 times the left-side of 
but the right-side of  is not 2 times the right-side of .
The given equations are a pair of parallel lines with no solution (since they do not intersect).