Question 289719
y=4x-18 
y=4x
----
These two lines do not cross; there is no solution for the system.
---------------------------------------
Substitute for "y" and solve for x:
4x = 4x-18
-18 = 0
That is a contradiction which resulted because we assumed
the x and y in the two equations have a consistent meaning.
===================
Cheers,
Stan H.