means x is 4
does not tell us what x or y are
If x=0, y=7
If x=1, y=6 etc
That's the scenario you have on the first line... 4x-2y=14.
If we knew what y is "in terms of x", we'd then have.... a certain number of x=value
The 2nd line achieves this situation if you use the given y back into the first line...
... use this y in the first line
Unfortunately, both of your equations are the equations of seperate parallel lines,
so substituting does not lead to a solution as there isn't any.
you can also see that there is no solution by basic manipulation.
Suppose, on the contrary, there was a solution x = a and y = b. Then 4a - 2b = 14 and b = 2a + 7. But 4a - 2b = 14 means 2(2a - b) = 14 which gives us 2a - b = 7.
So we have 2a - b = 7 and b = 2a + 7.
From the latter equation above, 7 = b - 2a = -(2a - b) = -7. But this is not possible! So our assumption that "a solution to our system of equations exists" must have been wrong. So we have no solutions for 4x-2y=14 and y=2x+7